Abstract:
A system includes an interface for receiving a pod having a carrier that receives wafers, and that is initially enclosed within a base and a pod cover. The system also includes a mechanism that transfers an exposed carrier between the interface and a platform of a mass-transfer machine included in the system. The machine includes a gantry arm for transferring the carrier between the platform and a transfer station. A retainer assembly is positionable over the carrier at the transfer station, and over a process carrier that is used in a processing tool. Moveable retainers of the assembly receive and hold wafers. The machine includes an elevator that moves between the transfer station and the process carrier. The elevator extends and retracts for transferring wafers between the retainers and either the carrier or the process carrier. A turntable, that receives the process carrier, permits automatically reorienting wafers.

Description:
CLAIM OF PROVISIONAL APPLICATION RIGHTS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/039,332 on Mar. 17, 1997, and is a continuation of application Ser. No. 09/038,809 filed Mar. 11, 1998, which issued Mar. 9, 1999, as U.S. Pat. No. 5,885,045. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to silicon wafer handling machines, and more particularly to systems adapted for automatically unloading silicon wafers from a Standard Mechanical InterFace (“SMIF”) pod and then transferring such wafers to a process carrier, and conversely. 
     2. Description of the Prior Art 
     Certain semiconductor wafer processing operations require that a number of disk-shaped silicon wafers be loaded into a process carrier arranged in a vertical orientation. Examples of such processes are “wet bench” processing and horizontal diffusion furnace processing. Presently, silicon wafers are transported between processing tools in a SMIF pod which orients the wafers horizontally. Accordingly, in addition to transferring wafers between the SMIF pod and the process carrier, performing any wafer processing operation in which the wafers must be oriented vertically requires reorienting the wafers from their horizontal orientation in the SMIF pod into a vertical orientation in the process carrier. In addition to reorientation of wafers between the SMIF pod and the process carrier, frequently the process carrier is capable of holding more wafers than the SMIF pod. Accordingly, in general preparing wafers for a process in which they are vertically oriented requires: 
     1. removing a SMIF pod&#39;s wafer carrier from within the protective environment provided by the SMIF pod; 
     2. removing the wafers from the SMIF pod&#39;s wafer carrier; 
     3. rotating the wafers from a horizontal to a vertical orientation either while they are present in, or after they are removed from, the SMIF pod&#39;s wafer carrier; 
     4. depositing the now vertically oriented wafers into a process carrier; and 
     5. perhaps performing the preceding operations more than once to combine wafers from more than one SMIF pod&#39;s wafer carrier into one process carrier. 
     To prevent contamination of silicon wafers during processing, present semiconductor processing technology requires that all of the preceding operations be performed automatically by a machine without human intervention in the process. Thus far, automation of this wafer handling process has been achieved by cascading a general purpose SMIF pod-load interface apparatus with a wafer mass-transfer machine with a process tool, e.g. a wet bench or a horizontal diffusion furnace. Assembling an entire apparatus for either of these process tools therefore results in two mechanical interfaces, i.e. the mechanical interface between the SMIF pod-load interface apparatus and the wafer mass-transfer machine, and the mechanical interface between the wafer mass-transfer machine and the process tool. Alignment of a mechanical interface, e.g. the mechanical interface between the SMIF pod-load interface apparatus and the wafer mass-transfer machine, can be so difficult that after the two devices have been disconnected, perhaps for repair or maintenance, several hours may be required to properly realign them. 
     In addition to the mechanical interfaces, there also exist electrical interfaces between the SMIF pod-load interface apparatus and a wafer mass-transfer machine, and the wafer mass-transfer machine with the process tool. In particular, the electrical interfaces between each of the devices must be arranged so the combined devices operate in a coordinated manner. Interfacing the SMIF pod-load interface apparatus with the wafer mass-transfer machine has proven to be troublesome and particularly annoying for process tool manufacturers desirous of selling an integrated system which includes the SMIF pod-load interface, the wafer mass-transfer machine, and the process tool. 
     In addition to the difficulties associated with interfacing the SMIF pod-load interface apparatus with the wafer mass-transfer machine, the combined devices occupy more floor space than desirable, and operate comparatively slowly because they are general purpose rather than special purpose devices. For example, a standard pod load interface opens a SMIF pod and transfers the wafer carrier to the process tool. For certain processes, the wafers must also be transferred from the original carrier to a different carrier. Under such circumstances, a wafer transfer machine has to be combined with a pod load interface to translate a carrier from a position within the pod load interface to a position within the wafer mass-transfer machine. For translating the SMIF pod&#39;s wafer carrier from one location to another location, generally the pod load interface includes an arm having at least two rotary joints which merely picks up the SMIF pod&#39;s wafer carrier, translates the carrier to a new location, and then set the SMIF pod&#39;s wafer carrier down. Accordingly, if the pod load interface is to also reorient the wafers from a horizontal orientation to a vertical orientation, an end-effector must be added to the standard pod load interface for performing the prescribed rotation. 
     In addition, the combined SMIF pod-load interface apparatus and wafer mass-transfer machine unnecessarily replicate certain subsystems. For example, a general purpose SMIF pod-load interface and a wafer mass-transfer machine each includes an environmental control system to prevent wafer contamination. Similarly, the SMIF pod load/unload device and wafer mass-transfer machine each include a separate electronic circuit for controlling their respective operation. 
     In addition to a horizontal orientation for the silicon wafers within the SMIF pod, it is often desirable to arrange the wafers with the backside of one wafer facing the frontside of the immediately adjacent wafer, or conversely. Generally, the backside of a silicon wafer is more likely to be contaminated than the wafer&#39;s frontside. Therefore, during wafer processing in which the wafers retain their SMIF pod&#39;s wafer carrier arrangement, contamination of the frontside of a wafer is more likely than if the wafers were arranged backside-to-backside and frontside-to-frontside. Such a rearrangement of the wafers into the more desirable backside-to-backside and frontside-to-frontside orientation is difficult to achieve with the combined SMIF pod-load interface apparatus and wafer mass-transfer machine. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system having higher throughput. 
     Another object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system that reduces mechanical interfaces. 
     Another object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system that eliminates electrical interface problems between the wafer transfer machine and the pod loader. 
     Another object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system having lower cost. 
     Another object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system that loads faster. 
     Another object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system that occupies less floor space. 
     Another object of the present invention is to provide an integrated wafer pod-load/unload and mass-transfer system that can provide back-to-back silicon wafer loading into a process carrier. 
     Briefly, the present invention integrates a SMIF pod loader, a wafer transfer machine, and mini-environment into a single system. For exchanging wafers between a carrier contained in a SMIF pod and a process carrier, the integrated wafer pod-load/unload and mass-transfer system links directly to a process tool. The pod load interface maintains an ultra clean environment for silicon wafers, and provides an ergonomic load port platform height for operator manual pod loading. A common operator panel is used to control all aspects of the process tool operation. The electronic controls are shared between several robotic elements. 
     The integrated pod-load/unload and mass-transfer system in accordance with the present invention automatically transfers silicon wafers between a SMIF pod and a wafer processing tool. As is known to those skilled in the art, the SMIF pod includes a wafer carrier adapted to receive a plurality of wafers. A base of the SMIF pod receives the wafer carrier and a SMIF pod cover mates with and seals to the base of the SMIF pod. In this way, the SMIF pod&#39;s cover and base completely enclose the SMIF pod&#39;s wafer carrier and any wafers carried therein. 
     The pod-load/unload and mass-transfer system itself includes a pod loader interface adapted to receive the SMIF pod, and either to expose or to reenclose the SMIF pod&#39;s wafer carrier. The pod-load/unload and mass-transfer system also includes a carrier load mechanism that is mechanically coupled to the pod loader interface. The carrier load mechanism transfers the SMIF pod&#39;s wafer carrier between a position in which the wafer carrier is exposed within the pod loader interface and a load platform. 
     Also included in pod-load/unload and mass-transfer system is a mass-transfer machine that includes the load platform. The mass-transfer machine, which is mechanically coupled directly to the pod loader interface, includes a gantry arm for transferring the wafer carrier between the load platform and a first wafer transfer station included in the mass-transfer machine. A retainer assembly, also included in the mass-transfer machine is positionable over either the SMIF pod&#39;s wafer carrier, when the wafer carrier is present at the wafer transfer station, and over a process carrier used in the wafer processing tool. The retainer assembly includes moveable retainers adapted for receiving and holding wafers. The mass-transfer machine includes at least one elevator moveable between positions in which the elevator is located either beneath the wafer transfer station or beneath the process carrier. The elevator extends and retracts for transfer-ring silicon wafers either between the wafer carrier present at the wafer transfer station and a position within the retainer assembly in which the retainers thereof may receive the wafers, or between the process carrier and a position within the retainer assembly in which the retainers thereof may receive the wafers. one embodiment of the present invention includes motorized turntable that receives the process carrier. The motorized turntable used in combination with the wafer elevators and the retainer assembly permits automatically reorienting the silicon wafers from a frontside-to-backside orientation to a backside-to-backside and frontside-to-frontside orientation. 
    
    
     These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of an integrated wafer pod-load/unload and mass-transfer system in accordance with the present invention depicting a pod load interface on which rests a SMIF pod, a SMIF pod&#39;s wafer carrier unloaded from the SMIF pod and resting on a load platform of a wafer transfer machine, and a mounting plate to which both the wafer pod-load/unload device and the mass-transfer machine are secured; 
     FIG. 2 is a front elevational view of the integrated wafer pod-load/unload and mass-transfer system taken along the line  2 — 2  in FIG. 1; 
     FIG. 3 is a plan view of the integrated wafer pod-load/unload and mass-transfer system taken along the line  3 — 3  in FIGS. 1 and 2; 
     FIG. 4 is the side elevational view of the integrated wafer pod-load/unload and mass-transfer system of FIG. 1 depicting a mini-environment for enclosing a SMIF pod&#39;s wafer carrier, two carriers loaded onto the wafer transfer machine, and is partially cut-away along a line  4 — 4  in FIG. 3 to illustrate operation of a carrier load mechanism; 
     FIGS. 5 a - 5   e  are schematic diagrams forming a sequence that illustrates transfer of a SMIF pod&#39;s wafer carrier from one location to another within the wafer transfer machine by a gantry included in the integrated wafer pod-load/unload and mass-transfer system; 
     FIGS. 6 a  through  6   c  respectively are side elevational, front elevational, and plan views of an alternative embodiment integrated wafer pod-load/unload and mass-transfer system with the front elevational view  6   b  being taken along the line  6   b — 6   b in FIGS. 6 a  and  6   c;    
     FIG. 7 is a plan view of the mounting plate depicted in FIG. 1; 
     FIG. 8 is a partially cross-sectioned front elevational view of the mounting plate taken along the line  8 — 8  in FIG. 7; and 
     FIG. 9 is a perspective view showing the retainer assembly and retainer arm which takes wafers in a horizontal array from two wafer carriers (each holding up to twenty-five wafers) at one loading platform and transfers the wafers to a larger carrier (holding up to fifty wafers) at another platform arranging the wafers in a backside-to-backside and frontside-to-frontside orientation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-4 depict different views of an integrated wafer pod-load/unload and mass-transfer system in accordance with the present invention referred to by the general reference character  20 . The system  20  includes a SMIF pod-load interface  22 , a wafer mass-transfer machine  24 , a mini-environment  26  and an L-shaped mounting plate  28 . As illustrated with dashed lines in FIGS.  2  and  3 , the system  20  abuts a process tool  32  which may be a “wet bench” for liquid immersion processing of silicon wafers, a horizontal diffusion furnace, or any other processing tool which requires vertically oriented wafers. The process tool  32  includes a robot arm (not depicted in any of the FIGS.) that positions a process carrier (also not depicted in FIGS. 1-4) onto the wafer mass-transfer machine  24  either to receive wafers from, or deliver wafers to, the system  20 . 
     As illustrated in FIGS. 1 and 4, the SMIF pod-load interface  22  receives a SMIF pod  36  onto a loading platform  38 . The pod load interface includes a pod present sensor (not separately depicted in any of the FIGS.) for detecting arrival or removal of a pod. The SMIF pod-load interface  22  includes a motorized pod opener mechanism (not separately depicted in any of the FIGS.). To open the SMIF pod  36 , the pod opener mechanism releases a SMIF cover  44  from a SMIF base  46 , and then raises the SMIF cover  44  above a SMIF pod&#39;s wafer carrier  48  carried within the SMIF pod  36  while concurrently enclosing the SMIF pod&#39;s wafer carrier  48  within the mini-environment  26 . An optical sensor (not separately depicted in any of the FIGS. and distinct from the pod present sensor described above) detects the presence of the SMIF pod&#39;s wafer carrier  48  in the SMIF pod  36 . When the SMIF cover  44  is raised, the SMIF pod&#39;s wafer carrier  48  remains within the mini-environment  26  to be thereby maintained in a class 1 environment. The SMIF pod&#39;s wafer carrier  48  in each SMIF pod  36  holds twenty-five (25) silicon wafers that are oriented horizontally. The SMIF pod-load interface  22  is similar to that described in U.S. patent application Ser. No. 08/400,039 filed Mar. 7, 1995, in the name of John Rush, that is entitled “Pod Loader Interface,” and that is hereby incorporated by reference. 
     A window  52 , that pierces a pod-loader-interface bulkhead  54 , permits a motorized carrier load mechanism  56  to access the SMIF pod&#39;s wafer carrier  48 . The carrier load mechanism  56  which is supported from the bulkhead  54 , and includes an end-effector  58  that rotates about a horizontal axis  62  to thereby enter through the window  52  into the mini-environment  26 . After entering the mini-environment  26 , the end-effector  58  engages the SMIF pod&#39;s wafer carrier  48 . The end-effector  58  then raises the SMIF pod&#39;s wafer carrier  48  off guides (not separately depicted in any of the FIGS.), and carrying the SMIF pod&#39;s wafer carrier  48  rotates in the reverse direction about the horizontal axis  62  thereby withdrawing out of the window  52  so wafers  64  in the SMIF pod&#39;s wafer carrier  48  become oriented vertically over the wafer mass-transfer machine  24 . The end-effector  58  then deposits the SMIF pod&#39;s wafer carrier  48 , about the center of gravity of the SMIF pod&#39;s wafer carrier  48 , onto a load platform  72  of the wafer mass-transfer machine  24 . Dedicating the carrier load mechanism  56  to transferring the SMIF pod&#39;s wafer carrier  48  between the SMIF pod-load interface  22  and the wafer mass-transfer machine  24  results in a simple mechanism that operates much more swiftly than previous systems. 
     Directly coupling the SMIF pod-load interface  22  to the wafer mass-transfer machine  24  reduces errors caused by mechanical interfaces between two independent units. The SMIF pod-load interface  22  and the wafer mass-transfer machine  24  also share common control electronics thereby eliminating potential software communications problems. 
     The wafer mass-transfer machine  24  transfers wafers  64  from SMIF pods&#39; wafer carriers  48  to a process carrier used in the process tool  32 . The wafer mass-transfer machine  24  includes a motorized gantry arm  76  that, as illustrated in FIGS. 5 a - 5   e , rises to pick-up the SMIF pod&#39;s wafer carrier  48  resting on the load platform  72 , and transports the SMIF pod&#39;s wafer carrier  48  horizontally away from the SMIF pod-load interface  22  to transfer stations  78  of the wafer mass-transfer machine  24 . The system  20  can be configured so the gantry arm  76  transports the SMIF pod&#39;s wafer carrier  48  different distances within the wafer mass-transfer machine  24  as required for compatibility with process carrier of the process tool  32 . 
     The SMIF pod-load interface  22  and the gantry arm  76  illustrated in FIGS. 1-4 may load one or preferably two SMIF pods&#39; wafer carriers  48  onto the wafer mass-transfer machine  24  at transfer stations  78  as illustrated in FIG.  4 . After the SMIF pods&#39; wafer carriers  48  are located in the transfer stations  78 , a motorized retainer assembly  82 , that is elevated above the SMIF pods&#39; wafer carriers  48 , moves horizontally across a fixed top plate  84  of the wafer mass-transfer machine  24  to a position over the SMIF pods&#39; wafer carriers  48 . Dual pedestal, U-shaped, motorized wafer elevators  86  then rise through the top plate  84  to lift the wafers  64  out of the SMIF pods&#39; wafer carriers  48  up to the retainer assembly  82 . If necessary, after the wafer elevators  86  raises the wafers  64  above the SMIF pods&#39; wafer carriers  48  but before elevating them to the retainer assembly  82 , a motorized indexing mechanism  88  moves the wafer elevator  86  furthest from the SMIF pod-load interface  22  horizontally toward or away from the SMIF pod-load interface  22 . Moving the wafer elevator  86  horizontally adjusts the position of the wafers  64  lifted out of the SMIF pod&#39;s wafer carrier  48  furthest from the wafer mass-transfer machine  24  to match the requirements of the process carrier of the process tool  32 . A pair of elongated motorized retainers  92  carried within and extending almost the entire length of the retainer assembly  82  then rotate under the wafers  64 , then supported on the wafer elevators  86 , to receive the wafers  64 . The wafer elevators  86  then retract downward beneath the top plate  84 , and the retainer assembly  82  now carrying the wafers  64  moves horizontally across the top plate  84  to position the wafers  64  over the process carrier of the process tool  32 . The wafer elevators  86  again rise to pick-up the wafers  64 , the retainers  92  then retract, and the wafer elevators  86  then descend to deposit the wafers  64  into the process carrier. The robot arm included in the process tool  32  then transfers the process carrier carrying the wafers  64  into the process tool  32  for processing. Operating in the manner described thus far, the system  20  may load up to fifty (50) wafers  64  at a time from two (2) SMIF pods&#39; wafer carriers  48  into a single process carrier. 
     After the wafers  64  undergo processing in the process tool  32 , a reverse sequence of operations removes the wafers  64  from the process carriers and stores them back into the SMIF pod  36 . 
     FIG. 7 illustrates the L-shaped mounting plate  28  upon which rest both an intermediate plate  94  for the SMIF pod-load interface  22 , and a base plate  96  for the wafer mass-transfer machine  24 . To facilitate radial alignment of the base plate  96  to the process tool  32 , the base plate  96  is secured to the L-shaped mounting plate  28  by threaded bolts  98  which pass through large apertures piercing the base plate  96  to screw into mating threaded holes in the L-shaped mounting plate  28 . A large washer  102  is interposed between the head of each bolt  98  and the base plate  96 . In a similar manner, the intermediate plate  94  is also secured to the L-shaped mounting plate  28  using bolts  98  passing through large apertures piercing the intermediate plate  94  and by large washers  102  that encircle the bolts  98 . Both the intermediate plate  94  and a base plate  95  of the SMIF pod-load interface  22  are joined together by guide pins  104  that fit into apertures piercing the intermediate plate  94  and the base plate  95 . The guide pins  104  ensure accurate repositioning of the SMIF pod-load interface  22  on the L-shaped mounting plate  28  after removal therefrom for repair or maintenance. 
     Referring now to FIG. 8, the L-shaped mounting plate  28  rests upon a pair of guide rails  103  that extend along sides of the L-shaped mounting plate  28 . A pair of stiffeners  105 , secured to the L-shaped mounting plate  28  beneath the L-shaped mounting plate  28 , extend from below the wafer mass-transfer machine  24  to below the SMIF pod-load interface  22  to support and stiffen that portion of the L-shaped mounting plate  28  which projects outward beyond the guide rails  103 . Supporting the L-shaped mounting plate  28  on the guide rails  103  permits sliding the entire system  20  forward or backward horizontally with respect to the process tool  32  to facilitate maintenance or repair. Bolts  108  lock the system  20  to the guide rails  103  during normal operation. The guide rails  103  rest upon and are secured to an interface plate  112 . The interface plate  112  in turn is supported from a frame  114  of the process tool  32  by four threaded jack screws  116 , only two of which appear in FIG.  8 . Four threaded bolts  118 , only two of which appear in FIG. 8, pass through apertures piercing the interface plate  112  to secure the interface plate  112  to the frame  114 . Adjustment of the jack screws  116  permits lowering the interface plate  112  toward or raising the interface plate  112  away from the frame  114 . In this way, the system  20  may be raised, lowered and tilted with respect to the process tool  32  both parallel to the process tool  32  and orthogonal to the process tool  32 . The adjustments permitted by this structure facilitate aligning the mechanical interface between the system  20  and the process tool  32  both radially and rectilinearly. 
     FIGS. 6 a - 6   c  depict an alternative embodiment of the system  20  that includes two (2) SMIF pod-load interfaces  22 . Those elements depicted in FIGS. 6 a - 6   c  that are common to the system  20  depicted in FIGS. 1-5 carry the same reference numeral distinguished by a prime (“′”) designation. The two (2) SMIF pod-load interfaces  22 ′ and carrier load mechanisms  56 ′ of the alternative embodiment system  20 ′ respectively transfer SMIF pods&#39; wafer carriers  48 ′ onto two (2) load platforms  72 ′ of the wafer mass-transfer machine  24 ′. Similar to the system  20 , a single gantry arm  76 ′ of the wafer mass-transfer machine  24 ′ appropriately position the SMIF pods&#39; wafer carriers  48 ′ horizontally with respect to the SMIF pod-load interfaces  22 ′ over the wafer elevator  86 ′. However, the system  20 ′ locates the wafers  64 ′ in the SMIF pods&#39; wafer carriers  48 ′ from one of the SMIF pod-load interface  22 ′ half-way between the wafers  64 ′ in the SMIF pods&#39; wafer carriers  48 ′ from the other SMIF pod-load interface  22 ′. 
     With the wafers  64 ′ from the two SMIF pod-load interfaces  22 ′ located half-way between each other, similar to the system  20 , the wafer elevator  86 ′ then lifts the wafers  64 ′ out of the SMIF pods&#39; wafer carriers  48 ′ up to the retainer assembly  82 ′. However, instead of a single pair of retainers  92  as in the system  20 , the retainer assembly  82 ′ of the system  20 ′ includes two (2) pairs of intermeshing retainers  92 ′ that are adapted to hold the wafers  64 ′ from both of the SMIF pod-load interfaces  22 ′ at a pitch, i.e. spacing between immediately adjacent wafers  64 ′, that is one-half of the pitch between immediately adjacent wafers  64 ′ in the SMIF pods&#39; wafer carriers  48 ′. In this way, the system  20 ′ combines on the retainer assembly  82 ′ the twenty-five (25) wafers  64 ′ from four (4) SMIF pods&#39; wafer carriers  48 ′ into a single group of one hundred (100) wafers  64 ′ for loading into a process carrier  122 . 
     As illustrated in FIG. 9, the mass-transfer machine  24 ′ may also include a motorized turntable  128  at one of three (3) transfer stations  78 ′ for reversing the direction of a larger wafer carrier  132  resting thereupon. As described in greater detail below, inclusion of the motorized turntable  128  in the mass-transfer machine  24 ′ permits automatic reorientation of wafers from two (2) SMIF pods&#39; wafer carriers  48  into a backside-to-backside and frontside-to-frontside orientation within the single wafer carrier  132 . To effect such a reorientation of the wafers, first two SMIF pods&#39; wafer carriers  48 ′ are deposited respectively onto two (2) of the transfer stations  78  from a SMIF pod-load interface, not depicted in FIG.  9 . After the SMIF pods&#39; wafer carriers  48 ′ are present on the transfer stations  78 ′, the motorized retainer assembly  82 ′, moves horizontally to a position above the SMIF pods&#39; wafer carriers  48 ′. Analogously to the description set forth above in connection with FIGS. 1-4, notorized wafer elevators then rise to lift the wafers out of the SMIF pods&#39; wafer carriers  48 ′ up to the retainer assembly  82 ′. Two pairs of elongated motorized retainers  92 ′, carried within and occupying almost the entire length of the retainer assembly  82 ′, then rotate under the wafers to receive the wafers. As described above, while the wafers  64  are being raised toward the retainer assembly  82 ′ the wafer elevators move closer together to match the pitch of all the wafers carried by the retainer assembly  82 ′ with the pitch of the wafer carrier  132 . The wafer elevators then retract downward beneath the transfer stations  78 ′, and both the elevators and the retainer assembly  82 ′ now carrying as many as fifty (50) wafers move horizontally across the mass-transfer machine  24 ′ to align with the wafer carrier  132 . 
     With the wafers now disposed in the retainer assembly  82 ′ over the motorized turntable  128 , a wafer elevator included therein rises to receive from alternating locations along the retainers  92 ′ as many as twenty-five (25) of the wafers, i.e. twelve (12) from one of the SMIF pod&#39;s wafer carrier  48 ′ and thirteen (13) from the other SMIF pod&#39;s wafer carrier  48 ′ or the converse. The elevator, carrying up to twenty-five (25) wafers, then descends into the mass-transfer machine  24  thereby depositing the wafers into the wafer carrier  132 . After the wafers are deposited in the wafer carrier  132 , the motorized turntable  128  rotates 180° so the frontsides of the wafers in the wafer carrier  132  face the backsides of the wafers still remaining above in the retainer assembly  82 ′. The wafer elevator carrying the now reoriented wafers again rises to the retainer assembly  82 ′ to receive the wafers remaining there. Carrying all the wafers now arranged in a backside-to-backside and frontside-to-frontside orientation, the elevator again descends into the mass-transfer machine  24 ′ to deposit the reoriented wafers into the wafer carrier  132 . A robot arm included in a process tool then transfers the wafer carrier  132  and the reoriented wafers into the tool for processing. As described above, organizing wafers backside-to-backside and frontside-to-frontside within the wafer carrier  132  for processing within the process tool eliminates transfer of contamination from the backside of one wafer to the frontside of the immediately adjacent wafer. 
     After the wafers have been processed in the tool, reversing the sequence of operations describe above transfers the wafers from the wafer carrier  132  back into the SMIF pods&#39; wafer carriers  48 ′ restoring all of the wafers to a uniform orientation. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is purely illustrative and is not to be interpreted as limiting. Consequently, without departing from the spirit and scope of the invention, various alterations, modifications, and/or alternative applications of the invention will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure.