Abstract:
A system for handling substrates held in a carrier, the system comprising a robot having an articulating robotic arm, a processor for controlling the robotic arm, an end effector attached to a moveable end of the robotic arm, the end effector comprising a blade having a first end and a second end, the blade having an active area for sensing a distance between the end and the substrate, and a passive gripper attached to the first end of the blade and an active gripper attached to the second end of the blade.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/411,372, filed on Sep. 16, 2002, the entire contents of which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to a substrate handling robot, and more specifically to a robotic arm end effector.  
         BACKGROUND  
         [0003]    Processing of a single semiconductor wafer often takes place in multiple fabrication facilities. Systems have been developed that are capable of sorting, tracking and packing/unpacking substrates to and from shipping containers. Such systems require transporting substrates such as semiconductor wafers into and out of the shipping containers. Systems for packing and unpacking substrate material often utilize vacuum wands and commercially available robotic systems. Such robotic systems can include one robotic arm assembly carrying multiple end effectors of different designs tailored to the needs of the specific substrate.  
           [0004]    There is a trend toward semiconductor wafer substrates becoming thinner due to packaging demands, improved thermal management, and a host of other reasons. Thinner substrates and the corresponding dimensional aspects of substrate cassette carriers impact the handling requirements of robotic arms and robotic end effectors. For example, wafer substrates are more likely to bow and warp when reduced in thickness. Wafer substrates including flats or fiducials add uncertainty to the location of the substrate within the cassette carriers. The overall physical characteristics of some silicon substrate wafers and the associated cassette holders can make precise determinations of position and orientation of the substrate difficult. Such substrate characteristics result in clearance issues when placed within substrate carriers which are difficult to overcome with available robotic end effector technology. Minimizing the end effector contact with the substrates reduces the possibility of contamination or damage. The total number of steps during substrate processing as well as the speed with which the robotic arm moves can limit the overall system throughput.  
         SUMMARY  
         [0005]    In one aspect, the invention features a system for handling substrates held in a carrier and having a robot including an articulating robotic arm, a processor for controlling the robotic arm, an end effector attached to a moveable end of the robotic arm, the end effector including a blade having a first end and a second end, the blade having an active area for sensing a distance between the end and the substrate, and a passive gripper attached to the first end of the blade and an active gripper attached to the second end of the blade. The substrate handled by the system can include, for example, a silicon wafer.  
           [0006]    In one embodiment, the end effector further includes a mapping sensor for detecting the mean vertical location of a substrate contained within the carrier. In another embodiment, the blade of the end effector is formed from a silicon wafer. In another embodiment, the blade is turned from a ceramic substrate. In various embodiments, the blade has a thickness less than about 1000 microns and preferably, less than about 750 microns. In one embodiment, the active area is formed on the blade from a metalization or thick layer process. In one embodiment, the active area is adapted to provide at least one of the mean vertical location, the thickness variation, the bow and warp, tilt, and deviation of the substrate within the substrate carrier. In one embodiment, the active area is a measurement transducer. In another embodiment, the active area is a capacitance probe. In various embodiments the active area includes at least one of optical sensor, pneumatic sensor, inductive sensor, ultrasonic sensor. In one embodiment, the active area includes at least three discrete sensors for providing planar information of the substrate.  
           [0007]    In one embodiment the active gripper is pneumatically actuated. In another embodiment the active gripper comprises a servo gripper coupled to a linear motor. In one embodiment the active gripper provides feedback to the processor for determining positive engagement with the substrate. In another embodiment, the active gripper provides feedback to the processor for determining the center of the substrate.  
           [0008]    In one embodiment, the invention features a prealigner including a prealigner chuck which is sized and configured to minimize contact with the surface of the substrate. In some embodiments, the prealigner chuck includes a plurality of projections or embattlements for supporting the substrate while it is rotated on the prealigner. In one example, the embattlements are sized and configured to allow full engagement of the grippers of the end effector with the substrate at any orientation of the a prealigner chuck. In one embodiment, the prealigner chuck includes a plurality of holes for optimizing the inertial properties and torque requirements of the chuck.  
           [0009]    In another aspect, the invention features a method for handling substrates held in a carrier including moving a robotic arm across an edge of the substrates, determining coordinate information of the substrates in the carrier, storing the coordinate information, sequentially indexing the robotic arm to the substrates in the carrier according the stored coordinate information, measuring a distance to the substrate from the arm, and engaging the substrate with robotic arm. In one embodiment, the coordinate information includes at least one of mean vertical location, the thickness variation, the bow and warp, tilt and deviation of the substrate within the substrate carrier.  
           [0010]    In another aspect, the invention features a method for handling substrates held in a cassette including providing a robotic arm including a mapping sensor and an end effector including a substrate sensor, moving the first sensor proximate to the cassette and recording the mean vertical substrate locations, generating a pick table including mean vertical substrate location data, sequentially indexing the robotic according to the mean vertical substrate locations of the pick table, engaging the cassette with the end effector, verifying the substrate position with the second sensor, and capturing and removing the substrate from the cassette with the robotic arm.  
           [0011]    In one embodiment, the generating of the mean vertical substrate location data is accurate to within 135 microns. In another embodiment, the recording of the mean vertical substrate location is accurate to within 100 microns. In one embodiment, the method includes prealigning the substrate after removing the substrate from the cassette. In another embodiment, the robotic arm includes an end effector having a blade with a first end and a second end, the blade including an active area for sensing a distance between the end effector and the substrate. In one embodiment, the end effector includes a passive gripper attached to the first end of the blade and an active gripper attached to the second end of the blade.  
           [0012]    In another aspect, the invention features a robotic end effector for holding a substrate and including a mapping sensor for detecting a mean vertical location of a substrate, a blade having a first end and a second end, an active area for sensing a distance between the end and the substrate located along the blade, and a passive gripper attached to the first end of the blade and an active gripper attached to the second end of the blade. In one embodiment, the active area of the end effector is formed from a metalization process. In another embodiment, the end effector includes a sensor for detecting the mean vertical location of a substrate. In still another embodiment, the active area of the end effector is at least three discrete sensors for providing planar information of the substrate. In various embodiments, the active area is adapted to provide at least one of the mean vertical location, the thickness variation, the bow and warp, tilt, and deviation of the substrate within the substrate carrier.  
           [0013]    In one embodiment the active area comprises a measurement transducer. In one embodiment, the active area includes a laser transducer. In another embodiment, the blade is turned from a silicon wafer. In still another embodiment, the blade is formed from a ceramic substrate. In various embodiments, the blade has a thickness less than about 1000 microns and preferably, less than about 750 microns.  
           [0014]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0015]    [0015]FIG. 1A is a front perspective view of a system for handling substrates.  
         [0016]    [0016]FIG. 1B is a partially exploded perspective view of the system of FIG. 1A with them enclosure panels removed.  
         [0017]    [0017]FIG. 2A is a rear perspective view of the system of FIG. 1A with the enclosure panels removed.  
         [0018]    [0018]FIG. 2B is an enlarged rear perspective view of the system of FIG. 1A.  
         [0019]    [0019]FIG. 3 is a perspective view of an articulating robotic arm.  
         [0020]    [0020]FIG. 4A is a top perspective view of an end effector attached to a portion of the robotic arm.  
         [0021]    [0021]FIG. 4B is a bottom perspective view of an end effector attached to a portion of the robotic arm.  
         [0022]    [0022]FIG. 4C is bottom perspective view of an end effector depicting the mapping sensor for scanning the substrates containing with cassette.  
         [0023]    [0023]FIG. 5A is a side perspective view of an end effector.  
         [0024]    [0024]FIG. 5B is a partially exploded perspective view of the end effector of FIG. 5A.  
         [0025]    [0025]FIG. 6A is a perspective view of an end effector engaging a substrate.  
         [0026]    [0026]FIG. 6B is a perspective view of the end effector of FIG. 4A with the guard box removed.  
         [0027]    [0027]FIG. 7 is a schematic side view of the end effector.  
         [0028]    [0028]FIG. 8 is a detailed schematic side view of an end effector.  
         [0029]    [0029]FIGS. 9A to  9 C are schematic views of a cassette holder for the storage and transport of substrates.  
         [0030]    [0030]FIG. 10 is a schematic side view of an end effector engaging the cassette holder of FIGS. 9A to  9 C.  
         [0031]    [0031]FIG. 11 is a detailed schematic view of a cassette holder containing substrates.  
         [0032]    [0032]FIG. 12 is perspective view of a prealigner.  
         [0033]    [0033]FIG. 13 is a side view of a chuck assembly for the prealigner.  
         [0034]    [0034]FIG. 14 is a flow chart representing exemplary process steps of a sorter application.  
         [0035]    [0035]FIG. 15 is a flow chart representing exemplary process steps of an operational sequence for calibration of the robotic arm sensors.  
         [0036]    [0036]FIG. 16 is a flow chart representing exemplary process steps of an operational sequence for a mapping the positions of substrates within a cassette holder.  
         [0037]    [0037]FIG. 17 is a flow chart representing exemplary process steps for engaging and gripping a substrate.  
         [0038]    [0038]FIG. 18 is a flow chart representing exemplary process steps for an operational sequence for centering and finding a fiducial of the substrate. 
     
    
       [0039]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0040]    Referring collectively to FIGS. 1A, 1B,  2 A and  2 B, a representative processing system  20  for handling substrates includes an enclosure  21  and an operation area  22  for the processing of a substrate  24  (FIG. 2B) including, for example, a semiconductor wafer. The enclosure  21  can include a drawer  26  for a keyboard and a graphical user interface  27 , such as a flat panel display secured to a mounting post  28 , for user input to a system computer (not shown). The enclosure  21  can also house a number of ancillary components including, for example, a power supply, a computer, pneumatic pump and controller, and storage (not shown). In one example, the system  20  is configured for complete processing of the semiconductor wafers  24  for routing through multiple fabrication facilities. As semiconductor wafers  24  have grown increasingly larger and thinner, the loading and unloading of the wafers  24  has become concomitantly more exacting.  
         [0041]    An important consideration in thin wafer design is that the substrate  24  is flexible and can be readily flattened and transported. The operation area  22  can include an articulating robotic arm  30  universal cassettes or input cassettes  34   a ,  34   b , an output cassette  38 , and a prealigner  40 . In one example, the system  20  includes a substrate scanner  41 . In one example, robotic arm  30  includes one or more end effectors  42  attached to an end of extender  44 . In one example, robotic arm  30  is a Robot ROB 310 and the end effector  42  is a flipper-type end effector.  
         [0042]    Referring to FIG. 3 and in one embodiment, the system  20  includes two robotic arms  30   a ,  30   b  each having 4-degrees of freedom. The robotic arms  30  can include one or more articulating arm linkages  46  and are attached to a mast  47  secured by a base  48 . End effectors  42   a ,  42   b  are coupled to the robotic arms  30  such that the end effector  42  can be rotated one-hundred-eighty degrees. The robotic arms  30  can be controlled by a Motoman ERC robot controller, for example. The robotic arms  30  are configured to move the end effectors  42  in radial, rotational and vertical directions, denoted by arrows R, θ, and Z, respectively.  
         [0043]    Referring collectively to FIGS. 4A, 4B,  4 C,  5 A,  5 B,  6 A, and  6 B the end effector  42  of the robotic arm  30  for the processing of substrate materials includes a blade or paddle  54  with first and second end grippers  56 ,  58  located along proximal and distal ends of the blade  54 , respectively. To minimize the thickness of the end effector  42 , in one example, the blade  54  is formed from a ceramic substrate. In one example, the blade is about 700 microns thick, about 3 inches wide, and about 8 inches long. In other examples, the blade  54  is formed from silicon, and more specifically, a semiconductor wafer. In one example, the first end gripper  56  is active, i.e., capable of movement with respect to the blade and the second end gripper  58  is passive, i.e., affixed along a distal end of the blade  54 . The blade  54  can be reinforced and attached to the guard box  80  with fastening strips  65   a ,  65   b  using fasteners  67   a ,  67   b  (FIG. 5B).  
         [0044]    In one example, the second end gripper  58  is constructed from high molecular weight plastic, such as PEEK® (Victrex Plc Corporation, Lancashire United Kingdom) material and the first end gripper  56  is made from a conductive material to discharge or prevent charge build-up of the substrate  24 . In one example, the second end gripper  58  is has an arcuate shape substantially conforming to the shape of the substrate  24  (shown in phantom in FIGS. 6A and  6 B) and is further sized and configured to substantially cover the distal (leading) end of the blade  54 . The second end gripper  58  can provide a shock absorption function to protect the blade  54  from damage during inadvertent impact of the leading end of the blade  54  with a hard surface. The first gripper  56  is can be actuated by a servo  76  coupled to a linear motor  78  (FIG. 6B), both housed within the guard box  80 . Alternatively, the first gripper  56  can be actuated pneumatically.  
         [0045]    The end effector  42  can include a plurality of sensors for sensing the location and mean position of the substrates  24 . The end effector  42  can be equipped with a mapping sensor  79  (FIG. 4C) disposed on the robot arm  30  and an active area  60  including substrate sensors  61  disposed directly on the blade  54 , in one example. The mapping sensor  79  can be a laser transducer, but other types of sensors are contemplated, including optical, inductive, and ultrasonic sensors, for example. An artifact (not shown) is located on the robotic arm  30  for a reference calibration point for both sensors  61 ,  79 .  
         [0046]    Once the blade  54  is cut to shape, a thick film or metalization process is performed on an outer surface of the blade  54  to form the active area  60  for detecting the presence and proximity of the substrate  24 . The thick film process can include a paste and oven process, for example. The thick film process permits the addition of the active area  60  directly on the blade  54  with substantially no increase in thickness to a surface of the blade  54 . In one example, the active area  60  is only about a few microns in thickness. The active area  60  can include a non-contact sensor transducer  61  such as a push-pull capacitive sensor, for example. The substrate sensor  61  can include guard rings (not shown) attached proximally thereto to increase durability. The substrate sensor  61  can be electrically connected to the system  20  via conductors located generally along areas  64  applied to the blade  54  using the thick film or metalization process used for forming the active area  60 . A ground plane (not shown) can also be layered onto the blade  54  using a thick film process between the substrate sensor  61 .  
         [0047]    In one example, the dynamic range of the substrate sensor  61  is about 4.0 mm and has a working stand off of about 0.8 mm. In one example, the substrate sensor  61  provides a TTL digital output and a ±5.0 VDC analog output for calibration. In one example, the substrate sensor  61  detects the capacitance or the amount of charge induced by the substrate  24  positioned proximally to the substrate sensor  61 . The system  20  is equipped with charge measuring circuitry (not shown) to determine the distance of the substrate  24  from the end effector  42 .  
         [0048]    The end effector  42 , shown schematically in FIG. 7 is rotatably attached to the robotic arm  30 . The first and second grippers  56 ,  58  define a critical plane Cp for a reference in calculating end effector  42  coordinates. A capture zone  68  is defined between the first and second grippers  56 ,  58  along the blade  54 . In operation, the robotic arm  30  rotates in the direction of arrow  62  one-hundred eighty degrees for displacement in one of two positions so that the end effector  42  engages the substrate  24  between the first end gripper  56  and the second end gripper  58  from either above or below the substrate  24 . In one example, the grippers  56 ,  58  contact the substrate  24  about less than 3 mm from an outer edge. The end effector  42  can accommodate generally circular substrates with diameters from about 100 to 200 mm, in one example. For versatility, the blade  54  can be readily replaced with blades of multiple sizes generally corresponding to the dimension of the substrate  24 .  
         [0049]    In one example, the movement of the active first end gripper  56  is pneumatically actuated and can also be spring loaded to provide from about 1 to 16 ounces of force, depending on the type of the substrate  24 . The grippers  56 ,  58  can include feedback sensors (not shown), such as monolithic transducer using silicon strain gauges to sense forces from engagement with the substrate  24 . The grippers  56 ,  58  can also be equipped with integral optical sensors (not shown) to indicate to the system  20  when the substrate  24  is engaged. The first end gripper can be equipped with land-surface datum (LSD) integral sensors to indicate the position of the gripper  56  along the blade  54 .  
         [0050]    With reference to FIG. 8, exemplary dimensions are depicted for the end effector  42 . In one example, the thickness T B  of the blade  54  is between about 0.4 mm to about 0.6 mm, and preferably about 0.5; the height of the grippers  56 ,  58  or a capture range R is between about 1.6 mm to about 2.0 mm, and preferably about 1.8 mm; and the clearance zone Z, the permissible distance between a top surface of the grippers  56 ,  58  and the next successive substrate, is between about 0.2 mm and 0.3 mm, and preferably about 0.25 mm. Other dimensions for the end effector  42  are contemplated for various applications and substrate sizes.  
         [0051]    Referring now collectively to FIGS. 9A to  9 C,  10  and  11 , the input cassette  34   a ,  34   b , in one example, includes a number of uniformly spaced slots  70  for supporting a number of substrates  24  in a generally horizontal configuration. The front of holder, as shown in FIG. 9B, includes a wide opening  72  for the loading and unloaded of the substrates  24 . The slots  70  are spaced to permit the passage of the end effector  42  between adjacent substrates  24 . The back of holder, as shown in FIG. 9C, includes a cutout  74  large enough to permit passage of the leading edge of the end effector  42 , as shown in FIG. 10. Generally, the robotic arm  30  is advanced along a Z direction (as denoted by the arrow in FIG. 10) until the end effector  42  corresponds to an interstitial space between the adjacent substrates  24  arranged in the input cassette  34   a ,  34   b . The robotic arm  30  is then advanced in an R direction (as denoted by the arrow in FIG. 10) until the end effector  42  passes under or over the particular substrate  24  that is to be removed from the input cassette  34   a ,  34   b . As the second gripper  58  proceeds past the cutout  74  to clear the edge of the substrate  24 , the robotic arm  30  advances slightly upward in the Z direction (or downward, if the substrate  24  is captured from above) to place the substrate  24  within the capture zone  68  (FIG. 7) between the first and second grippers  56 ,  58 . The first gripper  56  next moves in the R direction to capture the substrate between the grippers  56 ,  58 . The robotic arm  30  retracts from the input cassette  34   a ,  34   b  carrying the substrate  24 .  
         [0052]    Referring to FIG. 11, the substrates  24  are positioned within the slots  70  of the input cassette  34   a ,  34   b . Adjacent substrates are separated by a distance D s . The physical properties of the substrate  24  and the input cassette  34   a ,  34   b  are considered to determine the possible vertical zone occupied by the substrate  24  disposed in the input cassette  34   a ,  34   b . As the thickness and the spacing of the substrate  24  within the input cassette  34   a ,  34   b  decreases, requirements for handling the substrate  24  while concomitantly maintaining acceptable throughput increases. Multiple substrate  24  parameters can be considered when evaluating the requirements of the end effector  42  including the nominal thickness, thickness tolerance, thickness variation and bow and/or warp of the substrate  24  along the length. The pitch tolerance of the slots  70  of the input cassette  34   a ,  34   b  and the angle of the slot  70  with respect to vertical alignment of the substrate  24  can also be considered for determining the requirements of the system  20 .  
         [0053]    With continued reference to FIG. 11, the total possible vertical space T s  that can be occupied by the substrate  24  is determined by summing the center point thickness of the substrate  24 , the substrate thickness tolerance, the thickness variation the bow or warp amount, the pitch tolerance, the slot  70  angle with respect to the horizontal alignment (i.e., determinable movement of the substrate  24  within the slots  70  of the input cassette  34   a .  34   b  due to the angle of the slots). In a representative example, the T s  is about 1900 microns.  
         [0054]    Referring to FIGS. 12 and 13 and in one example, the prealigner  40  includes a prealigner chuck  90  (FIG. 13). The prealigner chuck  90  is sized and configured to minimize contact with the surface of the substrate  24 . For a substantially circular substrate  24 , an exclusion zone extends about 3 mm from the outside circumferential periphery of the substrate  24  wherein handling contact is permissible. The prealigner chuck includes a plurality of projections or embattlements  92  for supporting the substrate  24  while it is rotated on the prealigner  40 . The embattlements  92  can be sized and configured for asperity contact with the substrate  24 . In one example, six embattlements  92  are uniformly located around the outside circumferential periphery of the substrate  24 . In one example, the embattlements  92  are sized and configured to allow full engagement of the grippers  56 ,  58  of the end effector  42  with the substrate  24  at any orientation of the a prealigner chuck  90 . The prealigner chuck  92  can include a plurality of holes  94  for optimizing the inertial properties and torque requirements of the chuck  92 .  
         [0055]    In one example, the prealigner  40  is an Integrated Dynamics Engineer SPA  310  prealigner (sorter version) and includes a prealigner controller (not shown). The prealigner  40  can also include inspection capability such as a Cognex Insight 1700 vision system or an inspection station for detecting defects on the surface of the substrate  24 . The vision system can automatically adjust for differing diameters of the substrate  24 .  
         [0056]    In operation, the robotic arm  30  grasps a substrate  24  from either of the two input cassettes  34   a ,  34   b  and places the substrate  24  onto the prealigner  40 , if an identification reading of the substrate  24  is required. In some examples, the wafers  24  are placed into either the substrate shipper  38  or the two input cassettes  34   a ,  34   b  in predetermined orientations. In some examples, the substrate  24  is asymmetric and includes a “flat” or fiducial to provide a reference point and the prealigner  40  detects this asymmetry while rotating the substrate  24 . The prealigner  40  can then rotate the substrate  24  to a predetermined orientation as a function of the asymmetry. The robotic arm  30  picks up the substrate  24  by applying vacuum pressure at the end effector  42 , flips the substrate  24  upside down, and moves the substrate  24  over the substrate shipper  38 . The robotic arm  30  then releases the substrate  24  to allow the substrate  24  to float gently down onto the stack of wafers  24  in the substrate shipper  38 . A sensor (not shown) can be provided to check for a correct presence of an interleaf sheet  27  before releasing the substrate  24  into the substrate shipper  38 .  
         [0057]    Referring to FIG. 14, a process  100  for aligning and reading substrates  24  initializes ( 102 ) the system  20  and the system components. Process  100  selects ( 104 ) a particular job to execute and the corresponding materials which are loaded. Process  100  ( 106 ) maps the input cassette  34   a ,  34   a  with the mapping sensor  79  (described in more detail in connection with FIG. 16) for determining the location and position of the substrate  24  and for detected miscued substrates within the input cassettes  34   a ,  34   b . Process  100  picks ( 108 ) a substrate  24  from an input cassette  34   a ,  34   b  and rotates ( 110 ) the substrate  24  one-hundred-eighty degrees, if required. Process  100  places ( 112 ) the substrates on the prealigner  40  for, in one example, aligning ( 114 ) the substrate with a particular orientation in the θ direction for placement in the output cassette  38 . Process  100  reads ( 116 ) the substrate identification information while positioned in the prealigner  40 . Process  100  picks ( 118 ) the substrate  24  from the prealigner  40  and rotates ( 120 ) the substrate  24  one hundred eighty degrees, if required. Process  100  places ( 122 ) the substrate  24  in output cassette  38 . Process  100  determines ( 124 ) if additional substrates  24  require processing, and if necessary returns to picking ( 108 ) the remaining substrate  24  from the input cassettes  34   a ,  34   b . When all substrates  24  are processed, process  100  terminates ( 126 ).  
         [0058]    Referring to FIG. 15, a process  150  for calibrating the substrate sensor  61  and the mapping senor  79  initializes ( 152 ). Process  150  moves ( 154 ) the robotic arm  30  to measure the artifact with the mapping sensor  79 . Process  150  measures ( 156 ) the artifact with the robotic arm  30  at two or more different positions with respect to the artifact. Process  150  moves ( 158 ) the robotic arm  30  to measure the artifact with the substrate sensors  61 . Process  150  performs ( 160 ) a linear transformation and validates ( 162 ) the transformation by, for example, picking a substrate  24  from the input cassette  34   a ,  34   b . Process  150  then completes ( 164 ) the calibration.  
         [0059]    It should be understood that in some examples, the substrate sensor  61  and/or the mapping sensor  79  of the end effector  42  obviates the need for a separate process  100  for aligning the substrates  26  as the orientation of the substrates  24  is determined in the process  200  described as follows.  
         [0060]    Referring to FIG. 16, a process  200  for mapping the location of the substrates  24  positioned within the input cassettes  34   a ,  34   b  moves ( 202 ) the robotic arm  30  to face the mapping sensor  79  toward the input cassettes  34   a ,  34   b  at a first angle with respect to the critical plane C p  (FIG. 7). Process  200  enables ( 204 ) high resolution acquisition with the mapping sensor  79  for recording the output of the reflected mapping sensor  79 , at for example, 1 millisecond intervals. Process  200  moves ( 206 ) the robotic arm  30  in the Z direction up the open face  72  of the input cassettes  34   a ,  34   b . The mapping sensor  79  remains “on” until it clears a top edge of the substrate  24 .  
         [0061]    When the top of the input cassette  34   a ,  34   b  is reached, process  200  stops ( 208 ) the high resolution acquisition. In so doing, the mapping sensor  79  provides a thickness measurement of all substrates  24  in the input cassette  34   a ,  34   b . In some examples, due to the effects of latency, hysteresis, the speed of the robotic arm  30 , and the quality of the edge of the substrate  24 , the measured thickness of the substrate  24  may not be the actual thickness of the substrate  24 . Accordingly, process  200  moves ( 210 ) robotic arm  30  to face the mapping sensor  79  toward the input cassette  34   a ,  34   b  at a second angle and enables ( 210 ) high resolution acquisition with the laser sensor at this second angle. Process  200  moves ( 214 ) the robotic arm  30  in the Z direction down the open face  72  of the input cassette  34   a ,  34   b . The mapping sensor  79  remains on until it clears a bottom edge of the substrates  24 . Process  200  stops ( 216 ) the high resolution acquisition when the robot  30  reaches the bottom of the input cassette  34   a ,  34   b.    
         [0062]    With information acquired in the enabling  204  at the first angle and the enabling  212  at the second angle, a more accurate measurement of the position and orientation of the substrate  24  can be obtained. Process  200  generates ( 218 ) a pick table of the locations measured by the mapping sensor  79 , translated into end effector  42  coordinates (with respect to the critical plane C p ). The pick table can inform the robotic arm  30  if a substrate is double slotted (two substrates contained within a single cassette slot  70  or cross-slotted.) Generating the pick table requires a linear transformation between the mean position of each substrate  24  along the Z-axis and generating coordinates with respect to the critical plane C p . By considering the mean position of a first substrate along the Z-axis and a next substrate, a safe Z position for the end effector  42  to enter the input cassette  34   a ,  34   b  is determined by the linear transformation. After the pick table is generated in  218 , process  200  is concludes ( 220 ).  
         [0063]    Referring to FIG. 17, a process  300  for engaging and retracting a substrate  24  retracts ( 302 ) the robot  30  in the R-direction and moves the robot  30  in the Z and the θ directions to the first substrate pick position. Process  300  opens ( 304 ) the edge grippers  56 ,  58  of the end effector  42 . Process  300  extends ( 306 ) the robot  30  in the R-direction into the input cassette  34   a ,  34   a  (see FIG. 10, for example). Process  300  senses ( 308 ) the mean vertical position of the substrate  24  and moves ( 310 ) the robot  30  in the Z and R directions to position the substrate  24  in the center of the capture zone  68  of the end effector  42 . Process  300  activates ( 312 ) the grippers  56 ,  58  and the sensor  61  signal is checked ( 314 ) and the robot  30  moves ( 316 ) slightly upward in the Z-direction and retracts from the input cassette  34   a ,  34   b . Process  300  moves the robot  30  ( 318 ) positively in the θ and Z directions to place position, including for example, the output cassette  38 , for the substrate  24 . Process  300  moves ( 320 ) the robot  30  in the R-direction to the place position and moves the robot  30  ( 322 ) downward in the Z-direction to the place position for the substrate  24 . Process opens ( 324 ) the edge grippers  56 ,  58  of the end effector  42  and moves the robot  30  ( 326 ) in the R and Z directions and the sensor  61  signal is checked ( 326 ) and the robot  30  retracts ( 328 ) from the place position.  
         [0064]    Referring to FIG. 18, a process  400  for aligning a substrate  24  places ( 402 ) a substrate  24  upon the prealigner chuck  90  and commands ( 404 ) the prealigner ( 404 ) to locate the asymmetric flat or the fiducial of the substrate  24 . Process  400  rotates ( 406 ) the prealigner chuck  90  until the edge profile of the substrate  24  is ascertained. Process  400  then rotates ( 408 ) the substrate  24  for reading the substrate ID having a position which is known with respect to the fiducial. Process  400  reads ( 410 ) the substrate ID and returns ( 412 ) the substrate ID to the system  20 . Process removes ( 414 ) the substrate  24  from the prealigner chuck  90  and concludes.  
         [0065]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.