Abstract:
A system comprising a first robotic arm assembly for capturing and releasing a semiconductor wafer, a second robotic arm for capturing and releasing an interleaf, and a controller for actuation of the first and second robotic arms, the first and second robotic arms operating substantially simultaneously. At least one robotic arm can include a transfer arm, and a counterweight attached to a first end of the transfer arm and an end effector attached to a second, opposite end of the transfer arm. Two robotic arms permit the system to carry out the steps substantially simultaneously and therefore increase throughput significantly.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to U.S. Provisional Application Serial No. 60/405,568, filed Aug. 22, 2002, the entire contents of which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to a substrate processing system.  
         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 handling of semiconductor wafers as well as other packaging materials such as an interleaf materials into and out of the shipping containers. The interleaf material is placed between the wafers to protect and space the wafers and minimize contact between each wafer. Generally, wafers and interleafs are both placed horizontally into the shipping containers. The shipping container is typically a rigid open container that is slightly larger than the diameter of the substrate and is deep enough to support about twenty-five wafers per container.  
           [0004]    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 (e.g., wafer or interleaf).  
           [0005]    The packing and unpacking of wafers into and out of enclosures known as horizontal wafer shipping containers is a sequential process with one process step being conducted at a time. The number of steps and the speed with which the robotic transfer arm can process each step can determine the packing and unpacking speed and the system throughput.  
         SUMMARY  
         [0006]    In one aspect, the invention features a first robotic arm assembly for capturing and releasing a semiconductor wafer and having at least two degrees of freedom, a second robotic arm for capturing and releasing an interleaf and having at least two degrees of freedom, and a controller for actuation of the first and second robotic arms, the first and second robotic arms operating substantially simultaneously.  
           [0007]    In one embodiment, the second robotic arm includes a transfer arm having a first end and a second end, and mounted to a second arm base, a counterweight attached to the first end of the transfer arm, and an end effector attached to the second, opposite end of the transfer arm.  
           [0008]    In another embodiment, the end effector of the system is configured to apply variable pressure forces to capture and release the interleaf. In another embodiment, the end effector of the system is configured to sequentially apply negative and positive pressures to capture and release the interleaf. In another embodiment, the system further includes a sensor to detect a proximity and engagement of the interleaf with the end effector. In still another embodiment the sensor uses differential pressure, reflectance, imaging, capacitance or inductance to detect proximity and engagement of the interleaf.  
           [0009]    In another embodiment, the system includes a sensor to detect the material properties of the interleaf. In another embodiment, the sensor of the system uses differential pressure, reflectance, imaging, capacitance or inductance to detect the material properties of the interleaf. In another embodiment, the end effector arm further comprises electrodes to provide an electrostatic charge for capturing the interleaf. In another embodiment, the end effector is slidably disposed in a substantially vertical orientation at the second end of the transfer arm. In another embodiment, the end effector is configured to vertically actuate independently of the base. The robotic arms can be, for example, pneumatically actuated or actuated with electric servo motors. In one embodiment, the system includes an interleaf cassette holder having a pneumatic separator for separation of the interleafs.  
           [0010]    In another aspect, an assembly includes a transfer arm having a first end and a second end, the arm being mounted to a second arm base, a counterweight attached to the first end of the transfer arm, and an end effector attached to the second, opposite end of the transfer arm, the end effector configured to apply positive and negative pressures to a substrate. In one embodiment, the end effector of the system is configured to sequentially apply positive and negative pressures to the substrate. The robotic arm assembly can also include a sensor to detect the material properties of the substrate when coupled to the end effector.  
           [0011]    In another aspect, the invention features a method including providing a processing system having first and second robotic arms, the first robotic arm having a first end effector for capture and release of a semiconductor wafer, the second robotic arm having a second end effector for the capture and release of an interleaf sheet, positioning the second robotic arm such that the second end effector is proximate to interleaf sheets, applying a positive pressure through the second end effector to the interleaf sheets to separate an upper-most interleaf sheet from remaining sheets, applying a negative pressure through the second end effector to retain the upper-most interleaf sheet against the second end effector, and transporting the upper-most interleaf sheet from a first location to a second location, and releasing the upper-most interleaf sheet from the second end effector.  
           [0012]    In one embodiment, the interleaf sheet is released into a enclosure and the method further includes positioning the first robotic arm for capturing the semiconductor wafer from a first location and releasing the semiconductor wafer to a second location within a wafer shipper before each release of the upper-most interleaf sheet from the second end effector. In another embodiment, the method includes capturing the semiconductor wafer by applying negative pressure through the first end effector and releasing the wafer by applying ambient pressure through the first end effector.  
           [0013]    The systems and methods according to the invention provide at least improved interleaf transfer reliability and contamination control through the use of multiple robotic arm assemblies with at least one robotic arm including a Bernoulli end effector. This configuration allows performance of the steps substantially simultaneously rather than sequentially and therefore increase the system throughput significantly. One robotic arm assembly can be configured for each step of the sequence for enhanced speed and throughput. In one embodiment, one robotic arm is dedicated towards the handling of wafers and another robotic arm is dedicated to the handling of interleafs only. Each of the first and second robotic arms can be configured to handle either wafers or interleaf sheets.  
           [0014]    The system may be used to both pack and unpack the wafer shippers. Accordingly, identical systems may be located in both a shipping and receiving area of a semiconductor fabrication facility. The system also provides faster loading of semiconductor wafers than is possible with manual loading using an electrostatic or vacuum wand and affords the capability of loading wafers more reliably and with less breakage than manual loading. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0015]    [0015]FIG. 1 is a perspective view of a system for wafer processing.  
         [0016]    [0016]FIG. 2 is a perspective view of a top portion of the system of FIG. 1.  
         [0017]    [0017]FIG. 3 is a plan view of the system of FIG. 1.  
         [0018]    [0018]FIG. 4A is a side view of the transfer arm including an end effector of the system depicted in FIG. 1 positioned proximate to an interleaf cassette.  
         [0019]    [0019]FIG. 4B is a side view of the transfer arm of FIG. 4A engaging a cassette of interleafs.  
         [0020]    [0020]FIG. 5A is a detail view of the end effector of the transfer arm engaging an interleaf.  
         [0021]    [0021]FIG. 5B is a detail view of end effector of FIG. 5A withdrawing an interleaf from the cassette.  
         [0022]    [0022]FIG. 5C is a detail view of the end effector of FIG. 5A holding an interleaf.  
         [0023]    FIGS.  6 A- 6 C are schematic views of various embodiments of one end of the end effector of the transfer arm.  
         [0024]    [0024]FIGS. 7A and 7B are detail views of a wafer shipper.  
         [0025]    [0025]FIG. 8 is a flow chart representing exemplary process steps for the system of FIG. 1. 
     
    
       [0026]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0027]    Referring to FIG. 1, a processing system  10  includes an enclosure  15  and an operation area  20  for the processing of substrate materials including, for example, a semiconductor wafer  25  and an interleaf sheet  30 . The enclosure  15  can include a drawer  33  for a keyboard and flat panel display system having a graphical user interface (GUI) for user input to a system computer (not shown). The enclosure  15  can also house a number of ancillary components including, for example, power supply, a computer, pneumatic pump and controller, and storage (not shown). In one example, the system  10  is configured for complete processing of the semiconductor wafers  25  for routing through multiple fabrication facilities. As semiconductor wafers  25  have grown increasingly larger and thinner, the loading and unloading of the wafers  25  has become concomitantly more exacting. An important consideration in thin wafer design is that the wafer  25  is flexible and can be readily flattened and transported.  
         [0028]    Referring to FIGS. 2 and 3, the operation area  20  includes an articulated robotic arm (wafer arm)  35 , a transfer arm  40 , universal cassettes  45   a ,  45   b , an interleaf holder  50 , a horizontal wafer shipper  55 , and a prealigner  57 . The wafer arm  35  can be controlled by a Motoman ERC robot controller, for example. In one example, the prealigner  57  is an Integrated Dynamics Engineer SPA  310  prealigner (sorter version) and includes a prealigner controller (not shown). The prealinger  57  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 wafer  25 . The vision system can automatically adjust for differing diameters of the wafer  25 . In one example, the system  10  includes a wafer scanner (not shown). The horizontal wafer shipper  55  is also known as a “coin-pack” (CP) cassette or a “Jar”. In one example, the wafer arm  35  includes a wafer end effector  60  attached to an end of extender  65 . In one example, the wafer arm  35  is a Robot ROB  310  and the wafer end effector  60  is a flipper-type end effector.  
         [0029]    The system  10  can also include an ionization riffler  95  disposed on the operation area  20  adjacent to the interleaf holder  50  or an ionization riffler  97  disposed adjacent to the horizontal wafer shipper  55 . An interleaf recycle bin  98  (FIG. 1) can be attached to the enclosure  15  adjacent to the interleaf holder  50  and horizontal wafer shippers  55  for discarding the interleaves during unpacking of wafer shipper  55 .  
         [0030]    In one example, the wafer arm  35  removes a semiconductor wafer  25  from either of the two universal cassettes  45   a ,  45   b , by extending the wafer end effector  60  to engage a back side  75  of the semiconductor wafer  25 , using, for example, vacuum pressure to hold the wafer  25  for transport to the wafer shipper  55 . In one example, the transfer arm  40  uses a combination of vacuum and positive pressures to retrieve the interleaf sheet  30  from the interleaf holder  50  for transport to the wafer shipper  55 .  
         [0031]    In operation, the wafer arm  35  grasps a wafer  25  from either of the two universal cassettes  45   a ,  45   b  and places the wafer  25  onto the prealigner  57 , if an identification reading of the wafer  25  is required. In some examples, the wafers  25  are placed into either the wafer shippers  55  of the two universal cassettes  45   a ,  45   b  in predetermined orientations. In some examples, the wafer  25  is asymmetric and the prealinger  57  detects this asymmetry while rotating the wafer  25 . The prealinger  57  can then rotate the wafer  25  to a predetermined orientation as a function of the asymmetry. The wafer arm  35  picks up the wafer  25  by applying vacuum pressure at the wafer end effector  60 , flip the wafer  25  upside down, and moves the wafer  25  over the wafer shipper  55 . The wafer arm  35  then releases the wafer  25  to allow the wafer  25  to float gently down onto the stack of wafers  25  in the wafer shipper  55 . A sensor (not shown) can be provided to check for a correct presence of an interleaf sheet  30  before releasing the wafer  25  into the wafer shipper  55 .  
         [0032]    Sensors scan the wafer shipper  55  loaded with a stack of wafers  25  and interleaf sheets  30  to provide a stack height. The wafer arm  35  descends towards the wafer  25  and moves into slow mode with predefined steps as it approaches the back side  75  of the wafer  25 . Actuating the riffler  97  causes compressed air to flow from a number of apertures in the riffler and separates and floats a top wafer  25  up against the wafer end effector  60  so that when a vacuum is established between the wafer end effector  60  and the wafer  25 , the wafer arm  35  will move up and retract thereby retrieving the wafer  25 .  
         [0033]    Some examples of the system  10  may include a warning system during loading or unloading of a wafer shipper  55  to prevent damage to the wafers  25 . For example, if either the wafer or interleaf end effectors  60 ,  80  presses against the top of a stack and does not cause a detectable change in vacuum flow, continued lowering of either the wafer or transfer arms  35 ,  40  can damage or break a wafer  25 , and can also cause the stack or wafers  25  to bind together, making it difficult to remove the wafers  25 . In one embodiment, this warning condition is avoided by using a counter (not shown) that is decremented as each wafer  25  and/or interleaf sheet  30  is unloaded, the count usable to determine the remaining height of the respective substrates.  
         [0034]    When lifting the wafer  25  from the shipper  55 , a variable pause in the movement of the wafer arm  35  can be incorporated to allow any double wafer lifts to drift back after any static charges have bled away from the coupled wafers  25 . In some examples, this is a precaution followed even when utilizing the riffler  97 . In one example, the wafer arm  35  only lifts the wafer  35  a small distance before pausing. In one example, as the wafer arm  35  is approaching the stack, the riffler  97  is actuated when the wafer end effector  60  is approximately about 2 mm above the wafer  25  to prevent any wafers  25  from being displaced out of the wafer shipper  55 .  
         [0035]    The interleaf sheet  30  is then placed into the wafer shipper  55  with the wafer arm  35  or transfer arm  40 . The acquisition sequence for both wafers  25  and interleafs sheets  30  is the same. However, a difference between handling the wafers  25  and the interleaf sheets  30  involves a mid air transfer of the wafer  25  prior to placement of the wafer  25  in the wafer shipper  55  or returning the wafer  55  to the cassettes  45   a ,  45   b . The unpacking operation is the same as described above with the sequence is reversed.  
         [0036]    Referring to FIGS. 4A and 4B, the transfer arm  40  and interleaf holder  50  are shown. The transfer arm  40  defines two motion axes, Z i  and θ i , enabling vertical and rotational motion, respectively. The interleaf end effector  80  can be used for transferring both wafers  25  and interleaf sheets  30 . The interleaf end effector  80  is vertically deployed and makes contact with the a top surface of a substrate being transferred. The interleaf end effector  80  applies some type of grasping force that is greater than a weight of the substrate to be transferred. The technique used to develop the grasping force at the interleaf end effector  80  can be achieved in several different ways depending upon the type of substrate to be handled.  
         [0037]    In one example, the transfer arm  40  includes an interleaf end effector  80  attached to a first end of an extender  83  and a counterweight  88  is attached to a second, opposite end of the extender  83 . The transfer arm  40  can also include a linear bearing  90  to permit independent vertical movement of the end effector  80  for facilitating engaging and dropping interleaf sheets  30 . The transfer arm  40  can include an actuator base  93  to provide movement in the Z i  and θ i  directions. The actuator base  93  is connected to a controller  94  linked to the system computer (not shown). In various embodiments, the actuator base  93  is pneumatically actuated or utilizes electric servo motors having either direct drive or a transmission linkage to the transfer arm  40 .  
         [0038]    Typically, the system  10  includes position feed back sensors and/or switches (not shown) associated with the wafer arm  35  and/or the transfer arm  40  that are received as inputs to and controlled by the system computer. In one example, the transfer arm  40  is cushioned and can rotate through an arc of 135 degrees in the θ i  direction and has a vertical movement of about 3.5 inches in the Z i  direction. Other dynamic parameters for the transfer arm  40  can be used.  
         [0039]    In one example, a sensor is used to detect a change in the vacuum applied at the interleaf end effector  80 . Therefore, during operation, as the interleaf end effector  80  is lowered towards the interleaf sheets  30 , a detected change of a certain magnitude in a vacuum pressure level is recognized as indicating that the interleaf end effector  80  has contacted a top interleaf  30  in the stack. This detected change in vacuum causes an interrupt at the system computer, which, in turn, causes the system computer to raise the end effector  80  away from the interleaf stack  30 , and permit the transfer arm  40  to retrieve and retain only one interleaf sheet  30 . In this way, the vacuum change prevents the end effector  80  from pressing the interleaf sheets  30  together and minimizes the development of an electrostatic bond between multiple interleaf sheets  30 . This manner of picking up the interleaf sheets  30  is referred to as a “feather touch mode” material handling. The system  10  can also be configured so that the transfer arm  40  descends slowly towards the interleaf stack  30  and moves up relatively quickly to promote disengagement of the interleaf sheet  30  from the interleaf end effector  80  of the transfer arm  40 .  
         [0040]    A difficulty in handling substrates such as interleaves  30  and wafers  25  is adhesion that can develop between successive substrates caused by static forces. Using ionization rifflers  95 ,  97  to separate the interleaf sheets  30  and the wafers  25  in conjunction with the “feather touch mode” described above can be effective in reducing these static adhesion forces. The rifflers  95 ,  97  provide a flow or pressurized gas, air for example, through a number of holes in the rifflers  95 ,  97 , to the wafers  25  and the substrates  30  in a generally radially inward direction.  
         [0041]    In one example, the interleaf end effector  80  can be configured to cause a slight deformation of an interleaf  30  in order to further separate the interleaf sheets  30  and reduce electrostatic bonding between the interleaf sheets  30 . This can be accomplished, for example, by forming a curvature in a diffuser  10  attached to the interleaf end effector  80  and/or with the addition of outriggers  112  for deflection of the interleaf  30  following capture by the interleaf end effector  80 . The outriggers  112  can extend radially outward and downward along a circumferential surface of the diffuser  10  to deflect the interleaf sheets  30  upon engagement with the end effector  80 .  
         [0042]    Certain physical characteristics of the interleaf sheet  30  can make handling difficult. Specifically, a porosity of the interleaf sheet  30  lessens an effectiveness of the interleaf end effector  80 . In this case, it is possible to unintentionally pick up more than one interleaf sheet  30  at a time due to a vacuum bleeding through the interleaf sheets  30 . A wide variation in porosity of the interleaf sheet  30  prevents use of controlled vacuum flow to pick up single interleaf sheets  30  and use of the vacuum sensor to indicate that more than one interleaf sheet  30  has been picked up. One example addresses the porosity of the interleaf sheets  30  with a bellows type (not shown) pick up end effector  80  in conjunction with the use of the riffler  95 .  
         [0043]    In this way, it is be possible to “float” a top sheet the interleaf stack  30  up against a descending interleaf end effector  80  with a riffling action, and thereby pick up the floating interleave sheet  30  without having to lower the end effector  80  beyond a top interleaf sheet  30  (i.e., and pressing a top interleaf sheet  30  against the lower sheets in the interleaf stack  30 ). Further, the interleaf sheet  30  is generally extremely thin and flexible, making proximity detection more difficult. Accordingly, the system  10  can include an interleaf end effector  80  which is configured to both apply positive and negative pressure (vacuum) in a predetermined sequence to the interleaf sheet  30  through the end effector  80 . This permits very easily adjustable interaction forces (pressures) between the interleaf end effector  80  and the interleaf sheet  30 . This is particularly the case if the interleaf end effector  80  is configured such that it can interact with the often dynamically changing shape of the interleaf sheet  30 . In a preferred example, the end effector  80  of the transfer arm  40  first applies a positive pressure to a top interleaf sheet  30 . The positive pressure from the end effector  80  generates a lift force on the top interleaf sheet  30  such that the top interleaf sheet  30  separates from the interleaf stack  30  and rises to engage the diffuser  10  of the end effector  80 . A lift force that develops on the top interleaf sheet  30  in response to a positive pressure applied by the interleaf end effector  80  is described by a Bernoulli equation for fluid flow along a streamline. As the air flow from the interleaf end effector  80  and over the upper surface of the interleaf  30  is greater than the air flow over the lower surface of the interleaf  30 , the average pressure on the lower surface of the interleaf  30  is greater than the average pressure on the upper surface of the interleaf  30 . Accordingly, a net upward force, the lift force F (FIG. 5A) results.  
         [0044]    When the interleaf sheet  30  reaches and engages the diffuser  110 , a sensor determines a reduction in pressure to the transfer arm  40  and the system  10  next applies a vacuum to the interleaf sheet  30  to secure the interleaf sheet to the end effector  80  for transport. The use of this interleaf end effector  80  results in a highly reliable interleaf transfer mechanism independent of the porosity and geometry of the interleaf sheet  30 .  
         [0045]    In operation, the operation of the transfer arm  40  for the moving the interleaf sheet  30  to the wafer shipper  55  is described as follows. After picking up an interleaf sheet  30  the transfer arm  40  rotates over the wafer shipper  55  and releases the interleaf sheet  30  to allow it to float gently downward into the wafer shipper  55 . A sensor positioned adjacent to the wafer shipper  55  (or on one of the arms  35 ,  40 ) can be used to detect whether there is a wafer  25  present in the wafer shipper  55  before releasing the interleaf sheet  30 . Unloading of the interleaf sheet  30  can be accomplished by rotation of the transfer arm  40  for release into the interleaf recycle bin  98  (FIG. 1). In one example, during unloading of wafer shipper  55 , the riffler  97  can be used to separate the interleaf sheets  30  from the wafers  25 .  
         [0046]    Referring to FIG. 5A, the transfer arm  40  is moved in Z i  and θ i  (FIGS. 4A and 4B) directions to one of its end points in its travel range whereby a home switch (not shown) is sensed that sets a position of the transfer arm  40  with respect to a coordinate system of system  10 . The transfer arm  40  is then rotated to a “pick interleaf position”. At this point, the transfer arm  40  senses the vertical position of the interleaf sheet  30 . The transfer arm  40  then awaits further command. The transfer arm  40  is commanded to retrieve an interleaf  30 .  
         [0047]    This causes the interleaf end effector  80  to move downward into the interleaf holder  50 . Depending on the type of grasping force, a force is enabled as the interleaf end effector  80  travels to the pick up point of the next interleaf  30 . Referring to FIG. 5B, feedback sensing provides confirmation that an interleaf  30  has been positively captured by the end effector  80  and can be transferred to the wafer shipper  55 . Referring to FIG. 5C, the transfer arm  40  then retracts to a safe Z i  height and then rotates to the wafer shipper  55  and lowers the interleaf sheet  30  into the wafer shipper  55 . The grasping force is then removed and the interleaf  30  transferred to the shipper  55 . The interleaf end effector  80  of the transfer arm  40  can include a sensor (not shown) to detect material properties of the interleaf sheet  30  using, for example, differential pressure, reflectance, imaging, capacitance or inductance.  
         [0048]    As shown in FIG. 6A, the interleaf end effector  80  can include a large open diffuser  110  for providing negative or positive pressures to the interleaf sheets  30 . Alternatively, as shown in FIG. 6B, the interleaf end effector  80  can include a number of apertures  115  arranged in a configuration for optimized capture and release of an interleaf sheet  30 . In a further example, shown in FIG. 6C, the interleaf end effector  80  includes a number of electrodes  120  to provide an electrostatic charge for capturing the interleaf sheet  30 . The interleaf end effector  80  can include both the apertures  115  and the electrodes  120  for application of both pressure (negative and positive) and electrostatic forces for manipulation of the interleaf sheet  30 .  
         [0049]    As shown in FIGS. 7A and 7B, a number of semiconductor wafers  25  are carefully stacked into the wafer shipper  55  and an interleaf sheet  30  is placed between each wafer  25  as the wafers  25  are stacked into the wafer shipper  55 . The interleaf  30  protects the wafers  25  from physical or electrostatic damage that could occur during loading, unloading or transportation. Typically the interleaf  30  is made from anti-static material, such as a carbon fiber matrix, or for example, Tyvek®, sometimes referred to as “Tyvac”. The wafers  25  are typically placed face down in the wafer shipper  55  to accommodate vacuum handling by the wafer arm  35 . The wafer end effector  60  applies vacuum to the back side (or bottom)  75  of the wafer  25  to avoid damaging the top surface of each wafer  25  being handled. The wafers  25  can be further protected with top and bottom layers of open-celled foam  85   a ,  85   b  on the top and/or bottom of substrate stack. Each of the constituent components of the loaded wafer shipper  55 , the wafers  25 , the interleaf sheets  30  and the foam layers  85  are placed in a particular sequence before the cover  125  (FIG. 7B) is secured to the top of the wafer shipper  55  for transport.  
         [0050]    After the wafer shipper  55  is loaded with wafers  25  and interleaf sheets  30 , the stack cover  125  (FIG. 7B) is attached to the wafer shipper  55 , and several wafer shippers  55  can be placed in a shipping container (not shown) for bulk shipment. One advantage of the methods of system  10  are a possible reduction in wafer  25  breakage and a decrease in shipping volume reduction as compared to regular wafer shipping boxes (in some case, offering a reduction in shipping volume by an approximate factor of four). In other examples, the handling of wafers  25  and interleaf sheets  30  are handled with automated arms  35 ,  40  as described above, while the handling of the wafer shipper  55  and the respective cover  125  are performed manually.  
         [0051]    In some of the examples, one or more scanners are included in the system  10 . For example, a scanner (not shown) disposed on the wafer arm  35  can be used to scan the wafer shipper  55  for the position of the wafer, cross-slotted and double-slotted wafers. The scanner performs a preliminary scan on the wafer shipper  55  to check if the foam layer  85   a ,  85   b  is present before packing/loading of the wafer shipper  55 . The scanner can be used to store the height of the substrate stack. The scanner can check the height of the interleaf sheets  30  and provide a warning event such as, for example, a “low” warning which will allow one cassettes  45   a  or  45   b  to run, an “out” warning if the cassettes  45   a ,  45   b  are depleted such that there is not enough to run one more lot of wafers  25  and an “OK” if level is high. The warning events can be stored and the count decremented so scanning could be as required.  
         [0052]    In one example, the transfer arm  40  and the wafer arm  35  can run asynchronously with the transfer arm  40  checking status before releasing the interleaf sheet  30  and the wafer arm  35  checking status before moving over the wafer shipper  55 . In this example, the transfer arm  40  is configured to swing over the wafer shipper  55  but above the wafer arm  35  release position. In one example, system software can be configured to check position switches of the wafer and transfer arms  35 ,  40  to preclude potential collision of the wafer and transfer arms  35 ,  40 .  
         [0053]    Referring to FIG. 8, a process  130  for packing the wafer shipper  55  includes placing ( 135 ) the wafer shipper  55 , the interleaf holder  55  containing interleaf sheets  30  and the wafer cassettes  45   a ,  45   b  containing wafers  25  on the operation area  20 . If desired, an operator (not shown) can insert a layer of packing foam  85   b  into a base area of the wafer shipper  55  before filling the shipper with substrates  25 ,  30 . In some examples, after loading the above materials and holders, a system controller display (not shown) indicates what type and/or thickness of foam to place in the wafer shipping  55  before loading and to place on top of the substrates  25 ,  30  after loading and before placing a cover (FIG. 7B) on the wafer shipper  55  for shipping. In some examples, the system controller display can indicate a color code corresponding to the color of the required foam packing material  85   a ,  85   b .  
         [0054]    Process  130  loads ( 140 ) a lot number and/or other relevant information into the system  10  and checks ( 145 ) that the sizes of the wafers  25 , wafer shipper  55  and interleaf sheets are consistent. Process  130  presses ( 150 ) a run button which is illuminated on the flat panel display, for example, disposed within the drawer  33  (FIG. 1) on the enclosure  15  when the aforementioned steps are complete. Process  130  scans ( 155 ) the universal cassettes  45   a ,  45   b  with the wafer arm  35  for identification of filled slots, cross-slotted and double-slotted wafers. Process  130  scans ( 160 ) the height of the wafer shipper  55  and checks base line against stored information regarding base height with foam layer  85   b  in place and scans ( 165 ) the height of the interleaf sheets  30  in the interleaf holder  50 . This information can be obtained in the set up of the system  10  by scanning the wafer shipper  55 . The scanner checks the height of the interleaf sheets  30  in the interleaf holder  50 . The transfer arm  40  swings over interleaf holder  50  and descends and picks up interleaf sheet  30 . All systems go to a warning status.  
         [0055]    Process  130  retrieves ( 170 ) an interleaf sheet  30  from the interleaf holder  50  with transfer arm  40 . The transfer arm  140  rotates over the interleaf holder  50  and descends to retrieve an interleaf sheet  30  with interleaf end effector  80 . Process  130  releases ( 175 ) the interleaf sheet  30  from the end effector to float down into the wafer shipper  55 . The transfer arm  40  repeats the sequence for the next interleaf sheet  30 .  
         [0056]    At approximately the same time, the process  130  retrieves ( 180 ) the wafer  25  from the universal cassettes  45   a ,  45   b  with the wafer arm  25  and aligns ( 185 ) the wafer  25  by placing the wafer  25  on the prealigner  57  (if a wafer ID is required). The prealigner  57  aligns the wafer  25  and the wafer ID is read. If a remote camera is used, a robotic translation of the wafer ID is performed. The wafer arm  35  picks the wafer  25  from prealigner  57  and withdraws from the prealigner  57 .  
         [0057]    Process  130  inspects ( 190 ) the wafer  25  at the prealigner  57 . Wafers  25  found to have defects can be automatically classified and moved to the reject cassette (not shown) or manually classified and either a proceed or reject decision is made which would have some impact on throughput of the system  10 . Process  130  flips ( 195 ) the wafer  25  one-hundred-eighty degrees, by rotating the wafer arm  35 , and moving the wafer  25  towards the wafer shipper  55 . The system  10  checks the arms  35 ,  40  sequence and the interleaf/wafer sensors. Process  130  moves ( 200 ) the wafer arm  35  over the wafer shipper  55  at a predetermined height and releases ( 205 ) the wafer  25  to float into the wafer shipper  55 . Process  130  checks ( 210 ) whether all of the wafers  25  have been loaded into the wafer shipper  55  based on information retrieving during loading  140  and the number of wafers retrieved  180  and released  205 . The sequence of retrieving  170  through releasing  205  is repeated until all wafers  25  in the universal cassettes  45   a ,  45   b  are loaded into the wafer shipper  55 .  
         [0058]    After loading the wafer shipper  55 , process  130  scans ( 215 ) the height of the substrates  25 ,  30  and the foam layers  85   a ,  85   b  with the wafer arm  25  and measures the total height. The system  10  compares the measured height to a theoretical compressed stack height. Process  130  calculates ( 220 ) a requisite thickness of the foam layer  85   a  (FIGS. 7A, 7B) to complete the substrate stack and displays ( 225 ) a color code correspondign tot he foam layer  85   a . Process  130  moves ( 230 ) the wafer and transfer arms  35 ,  40  to safe locations to allow the wafer shipper  55  to be closed with cover  125  (FIG. 7B) and removed from the operation area  20 . If either an integrated or remote printer (not shown) is installed on the system  10 , process  130  prints ( 235 ) the wafer lot number and/or other data on an adhesive label (not shown) which is applied to the wafer shipper  55 .  
         [0059]    Unloading is generally the reverse of loading with the exception that when the wafer  25  is lifted from the wafer shipper  55 , there can be a variable pause to allow static electricity to bleed away from the wafer  25  before continuing with the unloading cycle as explained above. Unpacking the interleaf sheets  30  from the wafer shipper  55  is the reverse of loading in that the interleaf sheets  30  are returned to the interleaf holder  50 . The interleaf sheets  30  can also be jettisoned to the recycle bin  98  (FIG. 1) after the transfer arm  40  removes the interleaf sheet  30  from the wafer shipper  55 .  
         [0060]    The transfer arm  40  provides enhanced throughput of the system  10 . A rate limiting scenario can occur when the transfer arm  40  is just keeping pace with the movement of the wafer arm  35 . The wafer and transfer arms  35 , 40  can be mechanically tuned and the only software requirements entail reading the sensors before the next operation particularly in Z direction of the arms  35 ,  40 , i.e., before rotation, the system checks the position in the Z direction. The querying of the Z position sensors can be designed to minimize delays and maintain throughput of the system  10 . The actual retrieval of the interleaf sheet  30  is activated by a hardware interrupt. The sensor-detecting of wafers  25  and interleaf sheets  30  in wafer shipper  55  is active and will provide a warning and stop operation with the occurrence of any malfunction.  
         [0061]    The load position of the wafer arm  35  is beneath the transfer arm  40  and accordingly, the system  10  can be asynchronous up to the point where the wafer arm  35  is about to extend the wafer  25  over the wafer shipper  55 , whereupon the position of the transfer arm  40  and the interleaf vacuum status is checked against the interleaf/wafer sensor for the presence of the interleaf sheet  30  before proceeding to load the wafer  25 .  
         [0062]    A number of examples have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the system. For example, the wafer arm  35  and/or transfer arm  40  can be three-degrees of freedom articulating robotic arms (with movement possible in the Z,  0  and R directions, for example). The placement of the wafer and transfer arms  35 ,  40  can be modified upon the operation area  20  for a number of configurations. For instance, the wafer end effector  60  can be configured to engage and manipulate interleaf sheets  30  and the interleaf end effector  80  can be configured to engage and manipulate wafers  25 . Accordingly, other examples are within the scope of the following claims.