Abstract:
A high-efficiency buffer stocker is disclosed. The buffer stocker includes an overhead transport track for supporting overhead transport vehicles carrying wafer containers and at least one conveyor system or conveyor belt provided beneath the overhead transport track for receiving the wafer containers from the overhead transport vehicles on the overhead transport track. The buffer stocker is capable of absorbing the excessive flow of wafer containers between a processing tool and a stocker, for example, to facilitate the orderly and efficient flow of wafers between sequential process tools in a semiconductor fabrication facility, for example.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 13/646,755, filed on Oct. 8, 2012, which is a divisional application of U.S. patent application Ser. No. 11/431,646 filed on May 9, 2006, now U.S. Pat. No. 8,308,418 issued on Nov. 13, 2012, the entire disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to overhead buffer stockers in automatic material handling systems. More particularly, the present invention relates to a high-efficiency buffer stocker which is equipped with horizontal conveyors, vertical conveyors and/or horizontal carousels to expedite transportation of products among multiple locations in a facility. 
     BACKGROUND OF THE INVENTION 
     In the manufacturing of a product, the product is usually processed at many work stations or processing machines. The transporting or conveying of partially-finished products, or work-in-process (WIP) parts, is an important aspect in the total manufacturing process. The careful conveying of semiconductor wafers is especially important in the manufacturing of integrated circuit chips due to the delicate nature of the chips. Furthermore, in fabricating an IC product, a multiplicity of fabrication steps, i.e., as many as several hundred, is usually required to complete the fabrication process. A semiconductor wafer or IC chip must be transported between various process stations in order to facilitate various fabrication processes. 
     For instance, to complete the fabrication of an IC chip, various steps of deposition, cleaning, ion implantation, etching, and passivation must be carried out before an IC chip is packaged for shipment. Each of these fabrication steps must be performed in a different process machine, i.e., a chemical vapor deposition chamber, an ion implantation chamber, an etcher, etc. A partially processed semiconductor wafer must be conveyed between various work stations many times before the fabrication process is completed. The safe conveying and accurate tracking of such semiconductor wafers or work-in-process parts in a semiconductor fabrication facility is therefore an important aspect of the total fabrication process. 
     Conventionally, partially finished semiconductor wafers or WIP parts are conveyed in a fabrication plant by automatically-guided vehicles (AGVs) or overhead transport vehicles (OHTs) that travel on predetermined routes or tracks. For the conveying of semiconductor wafers, the wafers are normally loaded into cassettes or SMIF (standardized mechanical interface) pods and then picked up and placed in the automatic conveying vehicles. For identifying and locating the various semiconductor wafers or WIP parts being transported, the cassettes or pods are normally labeled with a tag positioned on the side of the cassette or pod. The tags can be read automatically by a tag reader that is mounted on the guard rails of the conveying vehicle. The AGVs and OHTs normally transport the pods from bay to bay along an interbay loop, and eventually deliver the pods to a robotic storage house, or “stocker”, which automatically delivers the pods to an intrabay loop. 
     In an automatic material handling system (AMHS), stockers are widely used in conjunction with automatically guided or overhead transport vehicles, either on the ground or suspended on tracks, for the storing and transporting of semiconductor wafers in SMIF pods or in wafer cassettes. For instance, as shown in  FIG. 1  of the drawings, three possible configurations for utilizing a stocker are illustrated. In case A, a stocker  10  is utilized for storing WIP wafers in SMIF pods and transporting them first to tool A, then to tool B, and finally to tool C for three separate processing steps to be conducted on the wafers. After the processing in tool C is completed, the SMIF pod is returned to a stocker  10  for possible conveying to another stocker. The configuration shown in case A is theoretically workable but hardly ever possible in a fabrication environment, since the tools or processing equipment cannot always be arranged nearby to accommodate the processing of wafers in the stocker  10 . 
     In the second configuration, case B shown in  FIG. 1 , a stocker  12  and a plurality of buffer stations A, B and C are used to accommodate three different processes to be conducted in tool A, tool B and tool C, respectively. As shown in  FIG. 1 , a SMIF pod may be first delivered to buffer station A from the stocker  12  and waits there for processing in tool A. Buffer stations B and C are similarly utilized in connection with tools B and C, respectively. The buffer stations A, B and C therefore become holding stations for conducting processes on the wafers. This configuration provides a workable solution to the fabrication process, but requires excessive floor space because of the additional buffer stations required. The configuration is therefore not feasible for use in a semiconductor fabrication facility. 
     In the third configuration, shown as case C in  FIG. 1 , a stocker  14  is provided for controlling the storage and conveying of WIP wafers to tools A, B and C. It is seen that after a SMIF pod is delivered to one of the three tools, the SMIF pod is always returned to to the stocker  14  before it is sent to the next processing tool. This is a viable process since only one stocker is required for handling three different processing tools and no buffer station is needed. The configuration shown in case C illustrates that the frequency of use of the stocker is extremely high since the stocker itself is used as a buffer station for all three tools. The accessing of the stocker  14  is therefore much more frequent than that required in the previous two configurations. 
       FIG. 2  illustrates a schematic of a typical automatic material handling system  20  that utilizes a central corridor  22 , a plurality of bays  24  and a multiplicity of process machines  26 . A multiplicity of stockers  30  are utilized for providing input/output to the bay  24 , or to the processing machines  26  located on the bay  24 . The central corridor  22  designed for bay layout is frequently used in an efficient automatic material handling system to perform lot transportation between bays. In this configuration, the stockers  30  of the automatic material handling system become the pathway for both input and output of the bay. Unfortunately, the stocker  30  frequently becomes a bottleneck for internal transportation. It has been observed that a major cause for the bottlenecking at the stockers  30  is the input/output ports of the stockers. 
     In modern semiconductor fabrication facilities, especially for the 200 mm or 300 mm FAB plants, automatic guided vehicles (AGV) and overhead transport vehicles (OHT) are extensively used to automate the wafer transport process as much as possible. The AGV and OHT utilize the input/output ports of a stocker to load or unload wafer lots, i.e., normally stored in wafer containers such as SMIF pods or FOUPs (front opening unified pods), for example. An overhead buffer (OHB) is typically provided near each process tool for the temporary storage of wafer containers prior to entry of each container into the process tool. 
       FIG. 3  is a perspective view of an overhead buffer (OHB)  32  including two vehicles  34 ,  36  that travel on a track  38 . Both an input port  40  and an output port  42  are provided on the stocker  30 . Each vehicle  36  stops at the input port  40  to place a wafer container  44  in the stocker  30 , while wafers (not shown) in the wafer container  44  await processing at a processing tool in the vicinity of the stocker  30 . An additional vehicle  36  either places an additional wafer container  44  in the input port  40  or retrieves a wafer container  44  from the output port  42  of the stocker  30 , depending on the availability of the next processing tool (not shown) in the fabrication sequence for processing of wafers contained in the wafer container  44 . 
     One limitation of the OHB  32  is that the OHB  32  is capable of accommodating only one vehicle  34  at a time. This causes considerable bottlenecking of multiple vehicles  34  at the input side or outlet side of the stocker  30 . Therefore, a high-efficiency buffer stocker is needed for absorbing and facilitating the orderly and efficient flow of multiple transport vehicles which transport wafer containers containing wafers to a stocker or from a stocker to a process tool. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to a high-efficiency buffer stocker for absorbing the excessive flow of multiple FOUPs or other wafer containers between a processing tool and a stocker, respectively, or between a stocker and a processing tool, respectively, to facilitate the orderly and efficient flow of wafers between sequential process tools in a semiconductor fabrication facility, for example. The high-efficiency buffer stocker includes an overhead transport (OHT) track which transports multiple wafer containers such as FOUPs, for example, to a stocker or process tool and a horizontal conveyor system which receives each of multiple wafer containers from an OHT vehicle on the OHT track, transports the wafer container along a horizontal transport pathway, and returns the wafer container to the OHT vehicle on the OHT track when the OHT track is capable of receiving the additional wafer containers. Multiple levels of the horizontal conveyors may be provided for receiving wafer containers from OHT vehicles located at various points along the OHT track and returning the wafer containers to various other points along the OHT track. The invention may further include a vertical carousel for receiving each wafer container from an OHT track, transporting each wafer container in a vertical transport path and returning the wafer container to the OHT track. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view illustrating three possible configurations for utilizing a stocker in a manufacturing facility; 
         FIG. 2  is a schematic view of a typical automatic material handling system which utilizes a central corridor, a plurality of bays and a multiplicity of process machines; 
         FIG. 3  is a perspective view of a conventional overhead transport vehicle (OHT) system; 
         FIG. 4A  is a top, partially schematic, view of a high efficiency buffer stocker according to one embodiment of the present invention; 
         FIG. 4B  is a perspective, partially schematic, view of the buffer stocker of  FIG. 4A ; 
         FIG. 5A  is a top, partially schematic, view of a high efficiency buffer stocker according to another embodiment of the present invention; 
         FIG. 5B  is a perspective, partially schematic, view of the buffer stocker of  FIG. 5A ; 
         FIG. 6A  is a top, partially schematic, view of a high efficiency buffer stocker according to still another embodiment of the present invention; 
         FIG. 6B  is a perspective, partially schematic, view of the buffer stocker of  FIG. 6A ; 
         FIG. 7A  is a top, partially schematic, view of a high efficiency buffer stocker according to yet another embodiment of the present invention; 
         FIG. 7B  is a perspective, partially schematic, view of the buffer stocker of  FIG. 7A ; and 
         FIG. 7C  is a side view of a carousel according to the embodiment of  FIGS. 7A and 7B . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention has particularly beneficial utility in the handling of wafer containers such as SMIF pods and FOUPs in semiconductor fabrication facilities. However, the invention is not so limited in application and while references may be made to such semiconductor fabrication facilities, the invention is more generally applicable to the transport and handling of materials in a variety of industrial and mechanical applications. 
     Referring initially to  FIGS. 4A and 4B  of the drawings, an illustrative embodiment of the high efficiency buffer stocker, hereinafter buffer stocker, of the present invention is generally indicated by reference numeral  50 . The buffer stocker  50  includes an overhead transport (OHT) track  52 . Multiple OHT vehicles  54 , which may be conventional, are mounted on the OHT track  52  for transport between process tools (not shown) or between a process tool and a conventional stocker (not shown) in a semiconductor fabrication facility, for example. Each OHT vehicle  54  is capable of carrying a wafer container  55 , such as a SMIF pod or FOUP, for example, which contains multiple wafers on which integrated circuits are being fabricated. 
     The buffer stocker  50  further includes an upper-level conveyor system  56 , a mid-level conveyor system  64  beneath the upper level conveyor system  56 , and a lower-level conveyor system  72  beneath the mid-level conveyor system  64 . The upper-level conveyor system  56 , mid-level conveyor system  64  and lower-level conveyor system  72  each typically includes a typically rectangular conveyor belt loop  57  having a pair of parallel longitudinal conveyor belts  62  and a pair of parallel transverse conveyor belts  63  disposed at respective ends of the longitudinal conveyor belts  62 . The upper-level conveyor system  56  includes a load/unload conveyor belt  58  which receives wafer containers  55   a  from OHT vehicles  54   a  that stop at a first position 1 on the OHT track  52 . The load/unload conveyor belt  58  transfers the wafer containers  55   a  onto the conveyor belt loop  57  of the upper-level conveyor system  56 . The mid-level conveyor system  64  includes a load/unload conveyor belt  66  which receives wafer containers  55   b  from OHT vehicles  54   b  that stop at a second position 2 on the OHT track  52 . The load/unload conveyor belt  74  transfers the wafer containers  55   b  onto the conveyor belt loop  57  of the mid-level conveyor system  64 . The lower-level conveyor system  72  includes a load/unload conveyor belt  74  which receives wafer containers  55   c  from OHT vehicles  54   c  that stop at a third position 3 on the OHT track  52 . The load/unload conveyor belt  74  transfers the wafer containers  55   c  onto the conveyor belt loop  57  of the lower-level conveyor system  72 . As shown in  FIG. 4A , the load/unload belt  58  of the upper level conveyor system  56 , the load/unload belt  66  of the mid-level conveyor system  64  and the load/unload belt  74  of the lower-level conveyor system  72  are located beneath the first, second and third positions  1 ,  2  and  3 , respectively, on the OHT track  52 . As shown in  FIG. 4A , a controller  51  is operably connected to the upper level conveyor system  56 , the mid-level conveyor system  64  and the lower-level conveyor system  72  for controlling the travel direction of each conveyor belt loop  57 . 
     In typical operation of the buffer stocker  50 , the OHT vehicles  54 , each of which carries a wafer container  55  containing wafers (not shown) on which integrated circuits are being fabricated, travel in either direction along the OHT track  52 . The OHT vehicles  54  typically transport the wafer containers  55  from a process tool (not shown) to a conventional stocker (not shown), or alternatively, from a conventional stocker to a downstream process tool. However, the conventional stocker is often filled to capacity with the wafer containers  55  or the process tool is often not available to receive an additional wafer container  55  for processing of the wafers therein. Therefore, bottlenecking of the OHT vehicles  54  on the OHT track  52  often occurs in the vicinity of the conventional stocker or process tool. 
     Accordingly, as shown in  FIG. 4B , as the OHT vehicles  54  travel along the OHT track  52 , when one of the OHT vehicles  54   a  reaches the first position 1 on the OHT track  52 , the wafer container  55   a  being carried by the OHT vehicle  54   a  can be lowered onto the load/unload conveyor belt  58  of the upper level conveyor system  56 . The load/unload conveyor belt  58  transports the wafer container  55   a  to the conveyor belt loop  57  of the upper level conveyor system  56 , as indicated by the load arrow  60 . The conveyor belt loop  57  carries the wafer container  55   a  along a transport pathway  59 . Meanwhile, the unloaded OHT vehicle  54   a  continues to move along the OHT track  52  to an alternative destination in the semiconductor fabrication facility to pick up and transport another wafer container  55  in the facility, for example. Multiple OHT vehicles  54   a , each of which carries a wafer container  55   a , may sequentially stop at the first position 1 to unload a wafer container  55   a  onto the load/unload conveyor belt  58 , in similar fashion. Accordingly, multiple wafer containers  55   a  may circulate simultaneously on the conveyor belt loop  57  of the upper level conveyor system  56 . 
     When space in the conventional stocker (not shown) becomes available for the storage of wafer containers  55 , or when the process tool becomes available for the processing of wafers in a wafer container  55 , one of the wafer containers  55   a  being transported by the upper level conveyor system  56  can be transported from the conveyor belt loop  57  back onto the load/unload belt  58 , as indicated by the unload arrow  61 , by reverse operation of the load/unload conveyor belt  58 . The wafer container  55   a  is then uploaded from the load/unload conveyor belt  58  and onto a vacant OHT vehicle  54   a  which is stopped at the first position 1 on the OHT track  52 . The OHT vehicle  54   a  then carries the loaded wafer container  55   a  to the conventional stocker, where the wafer container  55   a  is stored, or to the process tool, where the wafers in the wafer container  55   a  are processed. 
     As further shown in  FIG. 4B , in the event that the upper level conveyor system  56  becomes filled to capacity with wafer containers  55   a , additional OHT vehicles  54   b  can stop at the second position 2 on the OHT track  52  to unload wafer containers  55   b  onto the load/unload conveyor belt  66  of the mid-level conveyor system  57 . The load/unload conveyor belt  66  transports each wafer container  55   b  onto the conveyor belt loop  57  of the mid-level conveyor system  64 , as indicated by the load arrow  60 . The wafer containers  55   b  are transported by the mid-level conveyor system  64  along the transport pathway  59 , until the conventional stocker or process tool becomes available to receive a wafer container  55   b . At that time, one of the wafer containers  55   b  is transported from the conveyor belt loop  57  back onto the load/unload conveyor belt  66  and the wafer container  55   b  is uploaded from the load/unload conveyor belt  66  to a vacant OHT vehicle  54   b  stopped at the second position 2 on the OHT track  52 . The OHT vehicle  54   b  then transports the wafer container  55   b  to the conventional stocker or process tool. 
     As further shown in  FIG. 4B , when the upper level conveyor system  56  and mid-level conveyor system  64  have both become filled to capacity with wafer containers  55   a  and  55   b , respectively, additional OHT vehicles  54   c  can be stopped at the third position 3 on the OHT track  52 . A wafer container  55   c  is loaded from each OHT vehicle  54   c  onto the load/unload conveyor belt  74  of the lower level conveyor system  72 . The lower level conveyor system  72  transports the wafer containers  55   c  along the transport pathway  59  until the conventional stocker or process tool becomes available to receive a wafer container  55   c . A vacant OHT vehicle  54   c  stops at the third position 3 on the OHT track  52 , and one of the wafer containers  55   c  is transported from the conveyor belt loop  57  back onto the load/unload conveyor belt  74 . The wafer container  55   c  is uploaded from the load/unload conveyor belt  74  to the OHT vehicle  54   c , which then transports the wafer container  55   c  to the conventional stocker or process tool. 
     Referring next to  FIGS. 5A and 5B , another embodiment of the buffer stocker is generally indicated by reference numeral  82  and includes an OHT track  83  on which multiple OHT vehicles  84  are mounted for travel. Each of the OHT vehicles  84  carries a wafer container  88  for transport of the wafer container  88  between process tools or between a process tool and a stocker. The buffer stocker  82  further includes an upper level conveyor system  85  having a conveyor belt loop  86  which is typically rectangular in configuration. A lower level conveyor system  93  having a conveyor belt loop  94  is disposed beneath the upper level conveyor system  85 . A mid-level conveyor system  89  having a conveyor belt loop  90  may be provided between the lower level conveyor system  93  and the upper level conveyor system  85 . The upper level conveyor system  85  includes a load point  85   a  which is located beneath a first position 1 on the OHT track  83 , the mid-level conveyor system  89  includes a load point  89   a  which is located beneath a second position 2 on the OHT track  83 , and the lower level conveyor system  93  includes a load point  93   a  which is located beneath a third position 3 on the OHT track  83 . As shown in  FIG. 5A , a controller  91  is operably connected to the upper level conveyor system  85 , the mid-level conveyor system  89  and the lower-level conveyor system  93  for controlling the travel direction of the respective conveyor belt loops  86 ,  90 ,  94 . 
     The buffer stocker  82  is typically operated when a conventional stocker (not shown) serviced by the OHT track  83  becomes filled to capacity with the wafer containers  84  or when a process tool (not shown) serviced by the OHT track  83  is not available to receive an additional wafer container  84  for processing of the wafers therein. Accordingly, OHT vehicles  84   a  are stopped at a first position 1 on the OHT track  83  and wafer containers  88   a  are unloaded from the OHT vehicles  84   a  onto the load point  85   a  of the upper level conveyor system  85 . The wafer containers  88   a  are transported by the upper level conveyor system  85  along a transport pathway  87 , until one or more of the wafer containers  88   a  can be transported to the conventional stocker or process tool. Accordingly, each wafer container  88   a  is loaded from the load point  85   a  onto a vacant OHT vehicle  84   a  stopped at the first position 1 on the OHT track  83 . The OHT vehicle  84   a  then transports the wafer container  88   a  to the conventional stocker or to the process tool. 
     In the event that the upper level conveyor system  85  becomes filled to capacity with the wafer containers  88   a , additional wafer containers  88   b  can be loaded from OHT vehicles  84   b  stopped at the second position 2 on the OHT track  83  and onto the load point  89   a  of the conveyor belt loop  90 , and transported on the mid-level conveyor system  89 . In like manner, in the event that the mid-level conveyor system  89  becomes filled to capacity with the wafer containers  88   b , additional wafer containers  88   c  can be loaded from OHT vehicles  84   c  stopped at the third position 3 on the OHT track  83  and onto the load point  93   a  of the conveyor belt loop  94 , and transported on the lower level conveyor system  93 . When the conventional stocker or process tool becomes available for receiving wafer containers  84 , one or multiple wafer containers  88   b  can be loaded from the load point  89   a  of the mid-level conveyor system  89  and onto an OHT vehicle or vehicles  84   b  stopped at the second position 2 on the OHT track  83  and transported to the conventional stocker or process tool. Likewise, one or multiple wafer containers  88   c  can be loaded from the load point  93   a  of the lower level conveyor system  93  and onto an OHT vehicle or vehicles  84   c  stopped at the third position 3 on the OHT track  83  for transport to the conventional stocker or process tool. 
     Referring next to  FIGS. 6A and 6B , in still another embodiment of the invention the buffer stocker is generally indicated by reference numeral  98 . The buffer stocker  98  includes an OHT track  99  for transport of OHT vehicles  100 , each of which carries a wafer-containing wafer container  104 , between process tools (not shown) or between a process tool and a conventional stocker (not shown). The buffer stocker  98  further includes an upper-level conveyor belt  101 , a lower-level conveyor belt  103  and a mid-level conveyor belt  102  between the lower-level conveyor belt  103  and the upper-level conveyor belt  101 . As shown in  FIG. 6A , a controller  115  is typically operably connected to the upper level conveyor belt  101 , the mid-level conveyor belt  102  and the lower-level conveyor belt  103  for controlling the travel direction of each. 
     In operation of the buffer stocker  98 , multiple OHT vehicles  100  travel on the OHT track  99 , and each carries a wafer container  104  between process tools (not shown) or between a process tool and a conventional stocker (not shown). In the event that the conventional stocker or process tool is filled to capacity, an OHT vehicle  100   a  can be stopped at a first position 1 on the OHT track  99 . A wafer container  104   a  is then lowered from the OHT vehicle  100   a  onto one end of the upper-level conveyor belt  101 , which transports the wafer container  104   a  to the opposite end of the upper-level conveyor belt  101 . When the conventional stocker or process tool becomes available to receive wafer containers  104   a , the wafer container  104   a  can be loaded from the upper level conveyor belt  101  onto a vacant OHT vehicle  100   a  stopped at a fourth position 4 on the OHT track  99 . The OHT vehicle  100   a  then transports the wafer container  104   a  to the conventional stocker or process tool. 
     In the event that the upper level conveyor belt  101  becomes loaded to capacity with wafer containers  104   a , an OHT vehicle  100   b  can be stopped at a second position 2 on the OHT track  99  to unload a wafer container  104   b  onto one end of the mid-level conveyor belt  102 . The mid-level conveyor belt  102  then transports the wafer container  104   b  to the opposite end of the mid-level conveyor belt  102 , from which a vacant OHT vehicle  100   b  stopped at a fifth position 5 on the OHT track  99  can receive the wafer container  104   b  when the conventional stocker or process tool becomes available to receive the wafer container  104   b.    
     In the event that the mid-level conveyor belt  102  becomes loaded to capacity with wafer containers  104   b , an OHT vehicle  100   c  can be stopped at a third position 3 on the OHT track  99  to unload a wafer container  104   c  onto one end of the lower level conveyor belt  103 . The lower level conveyor belt  103  then transports the wafer container  104   c  to the opposite end of the lower level conveyor belt  103 . A vacant OHT vehicle  100   c  stopped at a sixth position 6 on the OHT track  99  is in position to receive the wafer container  104   c  when the conventional stocker or process tool becomes available to receive the wafer container  104   c . It will be appreciated by those skilled in the art that, by operation of the controller  115 , the direction of travel of the upper-level conveyor belt  101 , the mid-level conveyor belt  102  and the lower level conveyor belt  103  can be selected depending on the direction of travel of the OHT vehicles  100  on the OHT track  99 . 
     Referring next to  FIGS. 7A-7C , yet another embodiment of the overhead buffer stocker is generally indicated by reference numeral  105 . The buffer stocker  105  includes an OHT track  106  on which multiple OHT vehicles  107 , each of which carries a wafer container  114 , are mounted for travel between process tools (not shown) or between a process tool and a conventional stocker (not shown). At least one, and preferably, multiple carousels  108  are provided beneath the OHT track  106 . The carousels  108  are designated in  FIGS. 7A and 7B  as a first carousel  108   a , a second carousel  108   b  and a third carousel  108   c . Each carousel  108  typically includes a descending conduit  109 , a bottom transverse conduit  110 , an ascending conduit  111  and a top transverse conduit  112 . Multiple container support platforms  113  are mounted in the descending conduit  109 , bottom transverse conduit  110 , ascending conduit  111  and top transverse conduit  112 . A motor  116  ( FIG. 7C ) operably engages each of the container support platforms  113  for moving the container support platforms  113  throughout the carousel  108 , as indicated by the dashed arrows. A container opening  117  is provided in each top transverse conduit  112  for receiving a wafer container  114   a  into the carousel  108 , as hereinafter further described. As shown in  FIG. 7A , the container opening  117  of each carousel  108  is located beneath the OHT track  106 . As shown in  FIG. 7A , a controller  118  may be operably connected to the first carousel  108   a , the second carousel  108   b  and the third carousel  108   c  for controlling the travel direction of each. 
     In operation of the buffer stocker  105 , each of the OHT vehicles  107  carries a wafer container  114  between process tools or between a process tool and a conventional stocker. In the event that the conventional stocker or process tool becomes filled to capacity, OHT vehicles  107   a  can be stopped at a first position 1 on the OHT track  106 . Wafer containers  114   a  are unloaded from the OHT vehicles  107   a , through the container opening  117  and onto one of the container support platforms  113  in the first carousel  108   a . The container support platforms  113  are capable of transporting the wafer containers  114   a  through the first carousel  108   a  to position vacant container support platforms  113  beneath the container opening  117  to receive additional wafer containers  114   a . After the first carousel  108   a  has become filled to capacity with wafer containers  114   a , additional OHT vehicles  107   b  can be stopped at a second position 2 on the OHT track  106 . Wafer containers  114   b  can then be unloaded from each wafer container  114   b  and into the second carousel  108   b  through the container opening  117 . In the event that the second carousel  108   b  becomes filled to capacity with wafer containers  114   b , additional wafer containers  114   c  can be loaded into the third carousel  108   c  from OHT vehicles  107   c  stopped at a third position 3 on the OHT track  106 . 
     When the process tool or conventional stocker becomes available for receiving wafer containers  114 , a wafer container  114   a  can be unloaded from the first carousel  108   a  through the container opening  117  and back onto a vacant OHT vehicle  107   a  stopped at the first position 1 for transport of the wafer container  114   a  to the process tool or conventional stocker. In the same manner, the wafer containers  114   b  can be unloaded from the second carousel  108   b  onto vacant OHT vehicles  107   b  stopped at the second position 2, and the wafer containers  114   c  can be unloaded from the third carousel  108   c  onto vacant OHT vehicles  107   c  stopped at the third position 3. 
     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications may be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.