Patent Publication Number: US-6981832-B2

Title: Wafer handling system

Description:
REFERENCE TO RELATED APPLICATION 
   This application is a divisional of application Ser. No. 09/769,088, filed Jan. 24, 2001 (now U.S. Pat. No. 6,632,068, issued Oct. 14, 2003), which claims the benefit under 35 U.S.C. §119(e) of provisional Application No. 60/236,046, filed Sep. 27, 2000. 

   FIELD OF THE INVENTION 
   The invention relates to processing tools utilized for semiconductor wafer processing and more particularly to wafer processing tools that are configured for 300-mm wafer processing in which the wafers are supplied in FOUP cassettes. 
   BACKGROUND OF THE INVENTION 
   When processing semiconductor wafers in processing tools, the processing tool is adapted to and optimized for the size of the wafer to be processed. The size of the wafers are standardized in a limited number of discrete sizes where there has been a tendency towards increasing size in the past decades in order to increase the production efficiency. The three most recently introduced wafer sizes are 150 mm, 200-mm and 300-mm diameter. 
   A given wafer fabrication facility will normally be configured for handling one wafer size. However, in R&amp;D, pilot production or implementation of a new technology in production, there is a need for tools that can process multiple wafer sizes alternatingly or that can easily be reconfigured for handling different wafer sizes. In the past, when using open cassettes that hold the wafers, this was not a major issue. A wafer handling device can easily be configured so that it can handle wafers of different sizes from/to their respective open cassettes. However, the use of the closed FOUP cassettes for 300-mm wafers requires specific features in the wafer handling and cassette handling system that do not allow handling 200-mm open cassettes alternatingly in the same system. 
   Accordingly, a need exists for a system capable of handling multiple sizes of substrates in a state-of-the-art facility. 
   SUMMARY OF THE INVENTION 
   According to the invention a system, layed out for handling 300-mm wafers, supplied in 300-mm FOUP cassettes, is provided with a tool set to accommodate 200-mm cassettes during wafer transfer and cassette transfer. This tool set comprises a “Transhipment FOUP” cassette designed to receive a 200-mm open cassette in its interior. The term “Transhipment FOUP” is used herein as a label to distinguish 200-mm open cassettes and standard 300-mm FOUP cassettes. When 200-mm wafers need to be handled, the cassette handler places the 200-mm cassette in the Transhipment FOUP and, subsequently, the Transhipment FOUP, containing the 200-mm cassette, is brought into active connection with a wafer handling device. According to a further aspect of the invention, this tool set comprises a cassette handler adapter that allows the 300-mm FOUP cassette handler to handle 200-mm cassettes. The system is programmed such that the tools are automatically picked up from their storage position when needed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a 300-mm wafer processing system, constructed in accordance with the prior art. 
       FIG. 2  is a schematic plan view of the system of according to FIG.  1 . 
       FIG. 3  is a schematic plan view of a system constructed in accordance with a preferred embodiment of the invention. 
       FIG. 4  is a front/left/top perspective view of a Transhipment FOUP cassette, constructed in accordance with a preferred embodiment, without an open cassette. 
       FIG. 5  is a front/left/top perspective view of the Transhipment FOUP cassette of  FIG. 4 , having an open cassette positioned therein. 
       FIG. 6  is a front/bottom perspective of the Transhipment FOUP cassette of FIG.  5 . 
       FIG. 7A  is a schematic plan view of an input/output station of the system of FIG.  3 . 
       FIG. 7B  is a schematic rear elevational view of the input/output station of the system of FIG.  3 . 
       FIG. 8  is a top plan view of a cassette end effector, constructed in accordance with a preferred embodiment of the present invention. 
       FIG. 9  is a right elevational view of the cassette end effector of FIG.  8 . 
       FIG. 10  is a plan top view of a cassette handler adapter, constructed in accordance with a preferred embodiment of the present invention. 
       FIG. 11  is a right elevational side view of the cassette handler adapter of FIG.  10 . 
       FIG. 12  is a top plan view of a cassette handler in engagement with the cassette handler adapter of  FIGS. 10 and 11  and in engagement with a 200-mm cassette. 
       FIG. 13  is a right side elevational view of the cassette handler, cassette handler adapter, and 200-mm cassette of FIG.  12 . 
       FIGS. 14A-14D  are top plan and rear elevational views of a platform storing cassettes. 
       FIG. 15  is a top plan view of the platform of  FIG. 14  with a cassette sensor arrangement. 
       FIG. 16A  is a top plan view of a cassette transfer platform, constructed in accordance with a preferred embodiment of the present invention 
       FIG. 16B  is a front/right/top perspective view of the cassette transfer platform of FIG.  16 A. 
       FIGS. 17A and 17B  are right side elevational views showing the operation of loading an open cassette into the Transhipment FOUP of the preferred embodiments. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Initially, with reference to  FIGS. 1 and 2 , a wafer processing system according to the prior art is described. The subsequent figures and corresponding text below illustrate how the preferred embodiment is adapted to this particular wafer processing system. The skilled artisan will readily appreciate, however, that the principles and advantages of the cassettes described herein can be readily adapted to other processing systems where handling of cassettes and wafers therefrom is at least partially automated. 
     FIG. 1  shows, diagrammatically and partially exposed, a perspective view and  FIG. 2  shows a plan view of a wafer processing system for 300-mm wafers supplied 300-mm FOUP cassettes, according to the prior art. The wafer processing system is indicated in its entirety by reference numeral  1 . This system  1  comprises a housing  2  and is generally installed in a so-called “clean room.” In addition to the housing  2 , partitions  3 ,  4  and  5  are also present. The housing  2  delimits, with the partition  3 , a processing area or chamber  21 . In this example, the processing area  21  comprises vertical furnaces  6 ,  7 . The housing  2  and the partitions  3  and  4  define a wafer handling chamber  22 . A cassette transfer chamber  23  is defined between the housing  2  and partitions  4  and  5 . The input/output station, to transfer cassettes into and out of the system  1 , is indicated by reference numeral  33 . 
   Wafers  13  are supplied in cassettes  10 , which are placed on the input/output station  33 . A cassette handling device  31  transfers the cassettes  10  from the input/output station  33  through a closable opening  34  into a cassette store  8  located in the cassette transfer chamber  23 . The cassette store  8  is provided with a number of rotary platforms  27  above one another, on which the cassettes  10  are stored. The cassette handling device  31  is movable in vertical direction by means of an elevator  35  so that the different platforms  27  can be reached. The cassette handling device  31  is provided with a cassette end effector  32 , which has dimensions a little smaller than those of a series cut-outs  26  in the rotary platforms  27 . When the cassette handling device  31  has transferred a cassette into the store  8 , the end effector  32  can be lowered through one of the cut-outs  26  in one of the platforms  27  to place the cassette on the platform  27 . Subsequently, the cassette handler  31  can be retracted from cassette store  8 . The cassette handling device  31  is mounted such that it is able to transfer cassettes between the input/output station  33  and store  8 . The device  31  is also capable of transferring cassettes between the store  8  and a rotatable cassette transfer platform  30 , or between the input/output station  33  and the rotatable cassette transfer platform  30 . 
   The rotatable cassette transfer platform  30  is constructed such that, on rotation, the cassette is placed against the partition  4  between the cassette transfer chamber  23  and the wafer handling chamber  22 . The partition  4  is provided with a closure and a closure mechanism, together forming an interface schematically indicated by reference numeral  37 . The interface  37  is configured for 300-mm FOUP cassettes. After placing the cassette against the interface  37  in the partition  4 , the closure mechanism grips and unlocks the closure of the FOUP cassette and simultaneously opens the closure in the partition  4  and the closure of the FOUP cassette. 
   A wafer handling device  24  within the wafer handling chamber  22  transfers wafers between the cassette concerned and a wafer boat  12 . After completion of the loading of wafers into the wafer boat  12 , a boat transfer arm  16  moves the wafer boat  12  through a closable opening in partition  3  from the wafer handling chamber  22  into the processing chamber  21 . The processing chamber  21  is provided with a rotary boat transfer platform  11 , supporting the wafer boat  12 . Two reactors, which in this case comprise furnaces  6 ,  7 , are arranged in processing chamber  21 . The furnaces  6 ,  7  are positioned vertically and wafer boats, indicated by  12 , filled with wafers  13 , are introduced into the furnaces  6 ,  7  in the vertical direction from below. To this end, each furnace  6 ,  7  has an insertion arm  14 , which is movable in the vertical direction. Only one insertion arm  14  can be seen in FIG.  1 . 
   The treatment of a large number of wafers can be carried out as follows: The operator, shown diagrammatically in  FIG. 1 , loads the store  8  by introducing a number of cassettes  10  on the input/output station  33  and carrying out control operations on a control panel  36 . Each of the cassettes  10  is transferred from the input/output station  33  with the aid of the cassette handling device  31  into the storage compartments  9  made for these cassettes in the store  8 , specifically on the stacked rotary platforms  27 . By rotation of the store  8  and use of the elevator  35 , it is possible to fill various compartments with the cassettes  10 . After filling the store  8 , no further human interaction is required with this automated installation. 
   The cassettes  10  concerned are then removed from the store  8  by the cassette handler device  31  and placed on the cassette transfer platform  30 . The cassette transfer platform  30  comprises two levels, schematically indicated in  FIG. 1 , each level capable of receiving a FOUP cassette, where the two levels can be rotated independently of one another. Upon rotation of the cassette transfer platform  30 , the cassettes are placed against partition  4 . After opening of the closure of the FOUP cassette, together with the closure  37  in partition  4 , the wafers are removed by the wafer handler  24  and placed in a wafer boat  12 . After the wafer boat  12  has been filled, and becomes available for one of the reactors  6 ,  7 , the closure  19  in partition  3  is opened and the wafer boat  12  is placed on the rotary boat transfer platform  11  by the boat transfer arm  16 . The boat transfer platform  11  then moves the wafer boat  12  within the process chamber  21  to a position below the reactor to be loaded. Then the insertion mechanism or elevator  14  moves the boat into the reactor  6  or  7 . Treated wafers execute a movement counter to the above. 
   This system is described in further detail in published PCT application WO 99/38199 of applicant, the disclosure of which is incorporated herein by reference. Although an operator is described as introducing the cassette on the input/output station  33 , the system is designed such that the cassettes can alternatively be introduced on the input/output station by means of an automatic guided vehicle (AGV) or by an overhead hoist system. In such a case, the control system of the wafer processing system can be connected to a host computer system that carries out the control functions. In this way, no human interaction at the wafer processing system is required at all. 
     FIG. 3  shows a system according to  FIGS. 1 and 2  and in addition provided with the features of the present invention. Features corresponding to those of the previously described system are referenced by similar reference numerals. The system  1  is provided with an input/output station  33  for receiving a cassette, which station is configured so that it can selectively receive a 300-mm FOUP cassette  212  or a 200-mm open cassette  112 . Preferably, no mechanical adjustments are required when changing from one cassette type to the other. In the illustrated embodiment of the input/output station  33 , shown in FIG.  3  and described in more detail with respect to  FIGS. 7A and 7B , two input/output positions are provided in the station  33 , one configured to receive a 300-mm FOUP cassette  212 , and another configured to receive a 200-mm open cassette  112 . In an alternative embodiment, one input/output position can be provided that is capable of selectively receiving a 300-mm FOUP cassette or a 200-mm open cassette without requiring any mechanical adjustments. 
   Further, the system  1  is provided with a cassette transfer region adjacent the input/output station  33 . In the illustrated embodiment, the cassette transfer region is defined by the transfer chamber  23 , defined by the housing  2  and partitions  4  and  5 . The cassette transfer region thus includes a cassette store  8 , which can be loaded with a large number of cassettes for efficient access during processing without the need for continually retrieving cassettes from and returning cassettes to the clean room environment outside the system  1 . 
   The cassette transfer region also houses a cassette transfer mechanism. In the illustrated embodiment, the transfer mechanism includes a cassette handler  31  and a cassette transfer platform  30 . The cassette handler  31  includes an elevator  35  allowing access to the multiple rotary platforms  27  of the cassette store  8 , and serves to transfer cassettes among the input/output station  33 , the cassette store  8  and the cassette transfer platform  30 . The cassette transfer platform  30 , in turn, places cassettes into active communication with the interface  37  between the cassette transfer chamber  23  and the wafer handling chamber  22 , through which the wafer handling robot  24  can remove or replace wafers. In the illustrated embodiment, the cassette handler  31  transfers cassettes with the open side out, thus facilitating efficient storage on the round store  8  with narrow cassette ends facing the center of the rotary platforms  27 . The cassette transfer platform  30  is therefore configured to turn cassettes received from the cassette handler  31  until the open side faces the interface  37 . 
   Movements within the cassette transfer region are controlled by a cassette handling controller, including a central processing unit (CPU) and program therefor. In particular, the cassette handling controller instructs all movement of the cassette handler  31 , rotation of the rotary platforms within the cassette store  8 , rotation of the cassette transfer platform  30  and rotation of the input/output station  33 . Sensors (described below) are also connected to this controller. 
   The cassette handler  31  of  FIGS. 1 and 2  is provided with an end effector that is configured to carry the 300-mm FOUP cassettes. The preferred embodiment additionally allows the end effector  32  of the cassette handler  31  to handle a 200-mm open cassette  112 . For this purpose, the preferred system  1  is provided with a cassette handler adapter  400 , described in more detail below with respect to  FIGS. 8-13 . The cassette handler adapter is designed to be fitted on the bearing surface  32  of the cassette handler  31 , to accommodate a 200-mm open cassette, and to fit into a store for this adapter plate when it is not in use. 
   When a 200-mm open cassette  112  is to be handled, the cassette handling controller instructs the cassette handler  31  to automatically pick up the cassette handler adapter  400  from the store and, with the cassette handler adapter  400  on the bearing surface or end effector  32  of the cassette handler  31 , transfers the 200-mm cassette  112 . Instead of storing the cassette handler adapter at a storing or parking location, it is also possible that the cassette handler adapter be mounted permanently on the cassette handler. As a third option, the cassette handler adapter can be slidably or rotatably mounted on the cassette handler. In this third alternative, the cassette handler adapter can, for example, be hingedly attached to the cassette handler, such that the cassette handler adapter can be flipped or otherwise adjusted between an active position for handling 200-mm open cassettes and an inactive position that allows handling of 300-mm FOUP cassettes directly upon the cassette handler. 
   The preferred embodiments also provide structures and methods for selectively bringing either a 300-mm FOUP or a 200-mm open cassette into active connection with the wafer handling device in such a way that the sealed environments, used in a standard 300-mm FOUP approach, remain intact. For this purpose, the system  1  is provided with a so-called “Transhipment FOUP”  100  designed to receive a 200-mm cassette  112  in its interior, as described in more detail below with respect to  FIGS. 5  to  6 . Unlike standard FOUPs, the Transhipment FOUP  100  is not provided with a closure. The critical outside dimensions of the Transhipment FOUP  100  are substantially identical to those of a standard FOUP. 
   When 200-mm wafers need to be handled, the cassette handling controller instructs the cassette handler  31  to pick up the Transhipment FOUP  100  from its storage position and places the Transhipment FOUP on the cassette transfer platform  30 . Then the cassette handler  31  picks up the cassette handler adapter  400  ( FIGS. 8-13 ) from its storage position and, equipped with the cassette handler adapter, the cassette handler  31  picks up a 200-mm cassette  112  from a storage position. The handler  31  then transports and places the 200-mm cassette  112  in the Transhipment FOUP  100  (see FIG.  17 ). By rotation of cassette transfer platform  30 , the Transhipment FOUP  100  is placed against partition  4  and after opening of closure in the interface  37 , the Transhipment FOUP  100  containing the 200-mm cassette is brought into active connection with the wafer handler  24  within the handling chamber  22 . No special measures are required to allow the wafer handler  24  to handle 200-mm wafers from the 200-mm cassette. It suffices to program the wafer handler  24  with the positions of the center of the wafers and their mutual spacing, as is done for 300-mm wafer handling. 
   In processing chamber  21 , a 300-mm wafer processing furnace is provided above position  60  and a 200-mm processing furnace is provided above position  70  of the boat transfer platform or turntable  11 . The fact that in this case the processing chamber contains furnaces is not relevant to the invention. The processing area can comprise any kind of processing tool or can comprise one or more metrology tools instead of processing tools. In the latter case, the wafer processing consists of analysis of the wafer. It is also possible that processing chamber  21  is completely absent and a wafer sorting operation forms the only “treatment” that the wafers undergo. In that case the system will comprise at least two positions where a FOUP cassette can be brought in active connection with the wafer handler  24 . 
   The invention will be explained in further detail below referring to  FIGS. 4-17 . The Transhipment FOUP  100  for facilitating 200-mm wafer handling in the system  1  is detailed in  FIGS. 4  to  6 .  FIGS. 7A and 7B  illustrate adaptations to the input/output station  33 .  FIGS. 8  to  13  illustrate the wafer handler&#39;s end effector  32 , the cassette handler adapter  400  and their interaction to receive 200-mm open cassettes  112 .  FIGS. 14A  to  15  illustrate adaptations to the store  8 .  FIGS. 16A and 16B  illustrate adaptations to the rotatable cassette transfer platform  30 .  FIG. 17  illustrates placement of a 200-mm open cassette  112  within the Transhipment FOUP. Corresponding parts are indicated by like reference numerals throughout the figures. 
   In  FIG. 4 , showing in perspective view a Transhipment FOUP without a 200-mm open cassette, the Transhipment FOUP is indicated in its entirety by 100 and comprises a top plate  101 , a bottom plate  102  and sidewalls  103 . The sidewalls  103  are provided with windows  104 . The front side is provided with a flange surface  105 , adapted to be placed sealingly against partition  4 . In its interior, the Transhipment FOUP comprises a cassette receiving plate  110  to accommodate a 200-mm open cassette. The receiving plate  110  is mounted on legs  111  and is provided with cassette positioning fixtures  114  to horizontally fix the 200-mm cassette on the receiving plate  110 . The bottom mechanical interface of the Transhipment FOUP is preferably identical to that of a standard 300-mm FOUP, at least in features relevant to interfacing with other elements of the system  1 . Similarly, other critical outside dimensions of the Transhipment FOUP  100  fall within the dimensions of the envelope of a standard FOUP cassette. 
   The Transhipment FOUP is preferably also provided with a discharge pipe  121 , connected to an overpressure relief valve  120  that might become active when the Transhipment FOUP  100  is in active connection with the wafer handling chamber  22  (FIG.  3 ). The Transhipment FOUP  100  is not provided with a closure for the open side. Despite lack of a door, because its out surfaces otherwise match those of a standard FOUP, the feature is referred to herein as a “FOUP” to distinguish the 200-mm “open” cassette. 
   With reference to  FIG. 5 , the Transhipment FOUP  100  is shown accommodating a 200-mm “open” cassette  112  on the cassette receiving plate  110 .  FIG. 6  shows the outer surface of the bottom  102  of the Transhipment FOUP  100  with three positioning recesses  130 , intended to engage with three positioning pins, which pins are discussed below with respect to  FIGS. 8 ,  9 ,  16 A and  16 B. The position and the dimensions of these recesses  130  and pins are standardized for handling in the semiconductor industry. The recesses  130  form elongated grooves, extending in the radial direction. The combination of the recesses  130  with the positioning pins allows the Transhipment FOUP  100  to be self-centering on the pins, similar to a standard FOUP. 
   With reference to  FIGS. 7A and 7B , the preferred embodiment provides an input/output station  33  adapted to facilitate feeding 200-mm cassettes into the system  1  described above.  FIG. 7A  is a top down view and  FIG. 7B  is a rear view (seen from out of the system towards the input/output station  33 ). The input/output station  33  is provided with two cassette receiving positions  208  and  209 . Receiving position  209  is configured to receive 300-mm FOUP cassettes  212  and position  208  is configured to receive 200-mm open cassettes  112 . 
   Receiving position  209  is provided with a bearing surface  201 , which can be rotated around an axis  205  to make the cassette accessible for the cassette handler  31  (FIG.  3 ). The bearing surface  201  is provided with three positioning pins  202 , positioned and dimensioned according to SEMI STANDARD E 47.1 and the FOUP cassette is provided with matching recesses  130  (FIG.  6 ). Near each of these positioning pins  202 , a sensor  203  is provided to detect the presence of a FOUP cassette  212 . A plurality of positions  204  are also shown in  FIG. 7A  to indicate the points at which the positioning pins  302  of the cassette handler  31  contact a FOUP cassette  212  when lifting the cassette from the input/output station  33 . 
   The position  208  for receiving a 200-mm open cassette is designed as a position to receive a 300-mm FOUP cassette, as described above with respect to position  209 , with an additional a load port adapter frame  210  to accommodate the 200-mm open cassette. The load port adapter frame  210  is provided with a bottom mechanical interface identical to that of a 300-mm FOUP cassette so that it fits on the three positioning pins  202  of the 300-mm FOUP receiving position  209 . In this way, a receiving position can easily be reconfigured to the other cassette type by either removing or placing a load port adapter frame  210  from or onto a receiving position. The load port adapter frame  210  is provided with fixtures to fix the position of the 200-mm cassette  112  and with sensors (not shown) to detect the presence of a 200-mm cassette. In an alternative embodiment, a receiving position can be designed such that either a 200-mm cassette or a 300-mm cassette can be received at the same position and sensors can detect whether and of what size a cassette is present. 
   With reference to  FIG. 8  (top view) and  FIG. 9  (side view), the end effector  32  of the cassette handler  31  is shown. The end effector is indicated in its entirety by reference numeral  32  and is provided with a bearing surface  301  to carry 300-mm FOUP cassettes. The bearing surface  301  is provided with three positioning pins  302  to pick up a FOUP cassette. The positions of the positioning pins  302  correspond to those of the positions  204  ( FIGS. 7A and 7B ) of the input/output station  33  and are shifted inwardly as compared to the positions of positioning pins  202  (FIGS.  7 A and  7 B). Both positioning pins  202  ( FIGS. 7A and 7B ) and  302  can be accommodated in the positioning recesses  130  in the bottom plate  102  of the FOUP cassette because these recesses  130  extend over some distance in the radial direction. In this way, the FOUP cassette can be picked up by the inner positioning pins  302  on the bearing surface  301  of the cassette handler end effector  32  while it is supported by the outer positioning pins  202  of the input/output station  33 . 
   The end effector bearing surface  301  is provided with three sensors  303  to detect the presence of a FOUP cassette. Furthermore, the end effector bearing surface  301  is provided with a substantially rectangular hole  305  to detect the presence of a cassette handler adapter on the bearing surface  301  of the cassette handler and two substantially rectangular holes  304  for the detection of the presence of the presence of a 200-mm cassette on the cassette handler adapter, as will be explained in further detail below. 
   With reference to  FIG. 10  (top view) and  FIG. 11  (side view), a cassette handler adapter is provided in order to be able to handle 200-mm cassettes. The cassette handler adapter is indicated in its entirety by reference numeral  400  and is designed to be fitted on the bearing surface  301  of the cassette handler end effector  32  ( FIG. 8 ) and to receive a 200-mm open cassette on its bearing surface  401 . For this purpose the cassette handler adapter  400  is provided with three positioning holes  402  matching with the positioning pins  302  of the cassette handler end effector  32 . The cassette handler adapter  400  rests by gravity on the bearing surface  301  of the cassette handler end effector  32 . 
   When the cassette handler adapter  400  is placed on the cassette handler end effector  32 , sensors  303  are activated and the presence of an object on the bearing surface is detected. However, the sensors  303  cannot discriminate between the presence of a FOUP cassette and the presence of a cassette handler adapter  400 . To allow the unambiguous determination of the presence of the cassette handler adapter  400 , the bearing surface  401  of the cassette handler adapter  400  is provided with a bracket  405 , provided at one end with a downward pointing vane. The vane is not visible in the drawings because the cover  408  shields the position of the vane. When the cassette handler adapter  400  is placed on the bearing surface  301  of the cassette handler end effector  32 , the vane sticks through the rectangular hole  305  and activates a sensor mounted adjacent to the rectangular hole  305 , underneath bearing surface  301  and not shown in the drawings. In this way the presence of the cassette handler adapter  400  on the bearing surface  301  is detected. 
   Furthermore, the cassette handle adapter  400  is provided with two leaf springs  403 , fixed at one end and provided with a vane  404  at the other end. When a 200-mm open cassette is placed on the bearing surface  401  of the cassette handler adapter  400 , the upwardly pointing ends of the leaf springs  403 , provided with the vanes  404 , are pressed downwards through the rectangular holes  304 . Adjacent to the holes  304  are mounted sensors (not shown), which detect the presence of the vanes  404  and, hence, the presence of the 200-mm open cassette. 
   Furthermore, the bearing surface  401  is provided with fixtures  406  to horizontally fix the 200-mm open cassette thereon. The 200-mm open cassette rests by gravity on the bearing surface  401 . The cassette handler adapter  400  is also provided with a vertical bar  407  at the open side of the cassette to prevent the wafers from shifting out of the open cassette during cassette transfer. The bar is made of a material that does not damage or contaminate the wafers, like PEEK ®. A cover  408  covers one end of the cassette handler adapter  400 , adjacent the vertical bar  407 . 
   The cassette handler adapter  400 , with a bottom mechanical interface identical to that of a 300-mm FOUP cassette, can be stored, when not in use, at any storage location that is designed for storage of a 300-mm FOUP cassette. This can be any compartment or storage position  9  in the store  8 . In an alternative design, not shown in further detail, the cassette handler adapter can be made such that it is mounted permanently on the cassette handler, but such that it can be tilted or translated away when not is use. 
   In  FIG. 12  (top view) and  FIG. 13  (side view) the cassette handler end effector  32  is shown carrying the cassette handler adapter  400  while the cassette handler adapter  400  accommodates a 200-mm open cassette  112 . Features in  FIGS. 12 and 13  are referenced by the same reference numerals as for corresponding parts in  FIGS. 8-11 . The position of a wafer  13  is indicated in the cassette  112 . 
   In the store  8  for cassettes, a number of storage positions for 300-mm FOUP cassettes are similarly converted to 200-mm storage positions.  FIGS. 14A  to  14 D show a platform  27  of the store  8  from a top and side views.  FIGS. 14A and 14B  show a platform  27  for receiving 300-mm FOUP cassettes  212 . The platform  27  comprises four receiving positions, each position provided with three positioning pins  502 . In the preferred embodiment, each receiving position for a 300-mm FOUP cassette can be converted to two receiving positions for 200-mm cassettes, as shown in  FIGS. 14C and 14D . This can be achieved by providing a receiving position for a 300-mm FOUP cassette with a store adapter frame  503 , designed to fit on the 3 positioning pins  502  and to receive two 200-mm cassettes on its upper surface. Each position to receive a 200-mm cassette is provided with fixtures  506  to horizontally fix the 200-mm cassette. The core of the rotatable store  8  is provided with a clean air supply channel  510 , and filters  511 , not shown in detail, can be mounted at each 200-mm compartment of the platform  27 . By the use of store adapter frames  503 , the number of 200-mm storage locations can simply be increased or decreased by placing or removing one or more of the store adapter frames  503 . 
     FIG. 15  schematically shows the detection of cassettes in the cassette store  8  of  FIGS. 14A-14D . Adjacent to the cassette store  8 , a post  550  is mounted rigidly attached to the housing and extending in the vertical direction over the entire height of the cassette store  8 . On this post  550 , at each of the stacked platforms  27 , optical sensors  551  are mounted to detect the presence or absence of a cassette. Inside each storage compartment, a reflector is mounted on a leaf spring. The reflector is indicated by reference numeral  552  but not shown in detail. When no cassette is present, the light beam emitted by the optical sensor  551  is reflected back by the reflector  552  mounted on the leaf spring and the sensor  551  detects the reflected light. When a cassette is present, the bottom of the cassette presses downwards the leaf spring, the reflector  552  is moved out of the light beam and the emitted light is not reflected back to the sensor  551 . Consequently, the compartment is identified as being empty and available for loading a cassette only when the sensor detects a signal. The height of the sensors  551  on the post  550  with respect to the platform  27  and the direction in which the sensor is aligned are different for a 200-mm storage compartment as compared to a 300-mm storage compartment. The mounting of the post  550 , of the sensors  551  on the post  550 , and of the reflectors  552  relative to each other, are such that the presence of a cassette can be detected only when the cassette position on the rotary platform  27  is facing the cassette handler  31  (FIG.  3 ). This position, however, is the most relevant situation for detection. Although the system keeps track of where the cassettes in the system are, the sensors provide an extra check and safety in order to prevent, for example, attempts to place a cassette in a position where a cassette is already present. With this sensing strategy, each cassette position of the rotatable cassette storage platform  27  does not require an individual sensor  551 . This avoids the need for electrical wiring being routed via a rotation feed-through to a controller, which would increase the complexity of the system significantly.  FIG. 15  shows also in greater detail the clean air supply channel  510  and the air filter  511  for a 200-mm compartment. Furthermore, an air guiding strip  512 , extending over substantially the entire height of the filter, is provided to guide the air in the direction of the respective centers of the 200-mm cassettes. 
     FIGS. 16A and 16B  show the rotatable cassette transfer platform  30  from top down and perspective views, respectively. The rotatable cassette transfer platform  30  has a bearing surface  601 . The bearing surface  601  is provided with a cutout  606  to allow vertical passage of the end effector  32  of the cassette handler  31 . Three positioning pins  602  are positioned and sized to mate with the recesses  130  ( FIG. 6 ) in the bottom of a FOUP cassette and sensors  603  detect the presence of a FOUP cassette. Three sensors  603  are shown in FIG.  16 A. It will be clear that this provides a high degree of redundancy and safety and that a reduced number of sensors can also be used. The sensors  603  are not shown in FIG.  16 B. Opposite the open side of the cutout  606 , a sensor housing  610  is mounted on the bearing surface  601 . At the side facing the FOUP cassette, the sensor housing is provided with an opening  611  to pass a detection beam from and to a sensor, which mounted inside the sensor housing and not shown in the figures. When an empty Transhipment FOUP is placed on the bearing surface  601 , the sensors  603  detect its presence. However, the sensors  603  cannot discriminate between the presence of a standard FOUP and the Transhipment FOUP  100 . For this purpose the sensor inside sensor housing  610  is provided. The detection works similar to the detection of the presence of a cassette in the cassette store  8 , as described above. When a normal FOUP cassette is placed on the cassette transfer platform  30 , the light emitted by the sensor within the sensor housing  610  and passing through hole  611  will only be scattered diffusely, rather than reflected backwards in the direction of the sensor. Consequently, the sensor will not deliver a signal. 
   On the other hand, in case of a Transhipment FOUP  100  ( FIGS. 4-6 ) the sensor will deliver a signal in the following way: The cassette receiving plate  110  in the Transhipment FOUP  100  is provided with a reflector mounted on a leaf spring and properly positioned. When an empty Transhipment FOUP  100  is placed on the cassette transfer platform  30 , the light emitted by the sensor in the sensor housing  610 , passing through hole  611  and the window  104  in wall  103  of the Transhipment FOUP that faces the sensor housing, hits the reflector. The reflector reflects the light back in the direction of the sensor and the sensor within the sensor housing  610  will detect the reflected light. The sensors are connected to the controller. When sensors  603  and the sensor inside the sensor housing  610  are simultaneously activated, the presence of the Transhipment FOUP  100  on the platform  30  is unambiguously detected. When a 200-mm open cassette is subsequently placed on the cassette receiving plate  110  of the Transhipment FOUP, the leaf spring is pressed downwards and the reflector, mounted on the leaf spring, is moved out of the beam and the sensor does not detect a signal anymore. By this sequence of events the presence of a 200-mm cassette  112  inside the Transhipment FOUP  100  is detected. As can be seen from  FIG. 16A , the position of the cassette on the platform  30  is somewhat asymmetrical. 
   As noted above, movements within the cassette transfer region are controlled by a cassette handling controller, comprising a central processing unit (CPU) and associated programming. The controller communicates with the sensors described above and is programmed to have the cassette handler  31  automatically pick up (or otherwise install) the cassette handler adapter  400  when needed and to automatically store the cassette handler adapter  400  when not needed. Similarly, the cassette handling controller is programmed such that, when 200-mm wafers need to be handled from a 200-mm cassette, the cassette handler  31  automatically picks up the Transhipment FOUP  100  and places it on the cassette transfer platform  30  if not already there (as determined by sensor communications with the controller), and the handler  31  picks up the 200-mm open cassette and places it in the Transhipment FOUP  100 . The “need” for handling 200-mm open cassettes and the wafers stored therein can be pre-programmed or can be instructed by a user at the control panel  36  (see FIG.  1 ). 
   In operation, when one or more 200-mm open cassettes  112  need to be introduced into the system  1  the procedure is as follows: a 200-mm cassette  112  is placed on the 200-mm position  208  of the receiving station  33 . When the system  1  is instructed to handle the 200-mm cassette  112 , the cassette handler controller is programmed such that before handling the 200-mm cassette  112 , the cassette handler  31  automatically picks up the cassette handler adapter  400  from its storage position. Then the cassette handler  31 , equipped with the cassette handler adapter  400 , transfers the 200-mm cassette  112  from the receiving position  208  at the input/output station  33  to a 200-mm storage position in the cassette store  8 . Sequentially, more than one 200-mm cassette can be introduced according to the need of the moment. 
   When 200-mm wafers need to be loaded into a boat  12 , the cassette handler controller instructs the cassette handler  31  to transfer the Transhipment FOUP  100  from its storage position in the store  8  to the cassette transfer platform  30 . Then, the cassette handler  31  picks up the cassette handler adapter  400  from its storage position and, equipped with the cassette handler adapter  400 , retrieves a 200-mm cassette  112  (typically from the store  8 ) and places it on the cassette receiving plate  110  in the interior of the Transhipment FOUP  100 . Then the cassette transfer platform  30  rotates the Transhipment FOUP  100  and places it against partition  4 . After opening the closure of the interface  37  in partition  4 , the wafer handler  24  can transfer 200-mm wafers from the 200-mm open cassette  112  to a 200-mm wafer boat within the wafer handling chamber  22  for processing the wafers. The system is programmed such that it will load wafers only in a wafer boat corresponding with the size of the wafers to be loaded and the wafer boat will only be loaded into a furnace or other tool corresponding to the boat size (see FIG.  3 ). When transfer of the 200-mm wafers out of the cassette  112  is complete, the cassette handling controller instructs the cassette transfer platform to rotate the Transhipment FOUP  100  away from the interface  37  and the cassette handler  31  removes the 200-mm open cassette  112  from the Transhipment FOUP  100 , to make room for another cassette. 
   The procedure of placing a 200-mm cassette inside a 300-mm Transhipment FOUP is illustrated in FIG.  17 . The cassette handler  31 , equipped with the cassette handler adapter  400 , and bearing a 200-mm cassette  112 , moves into the Transhipment FOUP  100  at the appropriate height. When the cassette handler  31  has reached the correct distance into the Transhipment FOUP  100 , the cassette handler  31  moves downwardly through the cut-out  113  (see  FIG. 4 ) of the cassette receiving plate  110  till the 200-mm cassette  112  is placed on the cassette receiving plate  110 . Then the cassette handler  31  moves downward an incremental distance to release the 200-mm cassette  112  onto the plate  110  and the cassette handler  31  retracts from the Transhipment FOUP  100 . 
   To someone skilled in the art, it will be clear that the invention can be embodied in other ways. It is possible to replace the rotatable store by a bookshelf store wherein the compartments are arranged side-by-side according to a rectangular grid. Then the cassette handler requires corresponding degrees of freedom in movement to be able to reach all the compartments. It is also possible that the cassette handler brings the cassette in active connection with the wafer handler, with the use of a stationary platform instead of the rotatable platform according to the present embodiment. 
   The skilled artisan will similarly appreciate that many other omissions, additions and modifications to the preferred embodiment can be made without departing from the spirit and scope of the present invention. It is accordingly desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.