Abstract:
A method and system for transferring a wafer cassette between an Automated Guide Vehicle (AGV) and process equipment in a semiconductor fabrication facility involves transmitting a first signal from the AGV to the process equipment indicative of a cassette transfer request, checking whether the process equipment is in a suitable mode, setting the process equipment to the suitable mode after the checking step if the process equipment was not already in the suitable mode, and transferring the cassette between the AGV and the process equipment. A second transmitting step may be included, which sends a signal from the process equipment to the AGV authorizing the transfer of the cassette. The transmissions may be accomplished by wireless communications links such as a photo-coupled Parallel Input/Output link. A plurality of sensors may be used to detect whether a cassette is anywhere on a cassette stage during the checking step.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method for transferring wafer cassettes between semiconductor device process equipment and an Automated Guide Vehicle (AGV) at a semiconductor fabrication facility. More specifically, the present invention relates to a method that includes steps for checking the mode of the process equipment and changing the mode, if necessary, to a mode more suitable for the transfer that is to take place. 
     2. Description of the Related Art 
     The processes employed during the fabrication of semiconductor devices have become increasingly automated. Until recently, however, the transfer of semiconductor wafer cassettes among pieces of processing equipment has been a manual process involving a human operator. With the increasing integration of semiconductor devices, particles generated by the human body, formerly not considered to be a problem, have become a main cause of contamination. Consequently, much research and development has gone into automating the transfer of wafer cassettes. Automated Guide Vehicles (AGVs) have been developed to deliver cassettes under the remote control of a computer. Although great advances have been made, current systems still suffer some disadvantages. For example, if the process equipment is not in the correct mode when the cassette delivery unit arrives, the transfer may be initiated nonetheless. The result is that cassettes may not be transferred at all, or, worse, cassettes can be dropped and wafers spilled and broken. 
     FIG. 1 is a schematic diagram of a current wafer transfer system using communications links between an AGV and its controller and between the AGV and some semiconductor fabrication process equipment. The system includes transfer equipment  10 , process equipment  20 , and a host computer  30 . The transfer equipment  10  includes a controller  11  that itself is connected to and controlled by the host computer  30 . The transfer equipment  10  also includes an AGV  13  that operates under radio control by the controller  11  through a radio link represented by the arrow  12 . The process equipment  20  is simultaneously controlled by the host computer  30  through a second controller (not shown). The host computer  30  communicates with the transfer controller  11  and the process equipment controller (not shown) according to a Semiconductor Equipment Communication Standard (SECS). The AGV  13  communicates with the process equipment  20  through a communications link represented by another arrow  15 . In the current system, a photo-coupled Parallel Input/Output (PPIO) link serves as the communications link  15 . 
     A cassette transfer occurs when a wafer cassette (not shown) is loaded into the process equipment  20  from the AGV  13  or when a wafer cassette is unloaded from the process equipment  20  to the AGV  13 . The process equipment  20  includes a loading/unloading cassette stage  21  (FIG. 2) used when wafer cassettes are transferred between the process equipment  20  and the AGV  13 . 
     FIG. 2 is a schematic diagram showing the loading/unloading cassette stage  21  of the process equipment  20 . The stage  21  contains a sensor S 1  that can detect the presence of a cassette when the cassette is placed in a preferred location A on the stage  21 . 
     The operation of the current wafer cassette transfer system will be described with reference to FIG.  1  and FIG.  2 . Starting with FIG. 1, a command to transfer a wafer cassette is issued by the host computer  30  which communicates the command through the connections to the transfer equipment controller  11  and to the controller (not shown) of the process equipment  20  according to SECS. The transfer equipment controller  11  then utilizes the radio link  12  to instruct the AGV  13  to move toward the process equipment  20 . As a result the AGV  13  arrives at the process equipment  20  adjacent to its loading/unloading cassette stage  21 . 
     After arrival at the process equipment  20 , the AGV  13  begins to communicate with the process equipment  20  via the communications link  15 . In the case of the PPIO link, the communications protocol is defined by the Semiconductor Equipment and Material International (SEMI) standard. The process equipment  20  has internal mechanisms (not shown) that must be deployed in a certain fashion for loading or unloading operations. For example, the process equipment  20  may have a door that must be opened, or the process equipment  20  may have a robot arm that must be unfolded during loading and unloading. In addition, the cassette stage  21  must be occupied by a cassette to allow unloading to occur, or the stage  21  must be empty to allow loading to occur. The state of such various internal mechanisms and the state of occupancy of the stage  21  constitute the transfer mode of the process equipment  20 . In the conventional wafer transfer system of FIG. 1, the process equipment  20  is considered to be in a mode suitable for the transfer as commanded by the host computer  30  when the AGV  13  arrives and begins communicating, regardless of whether the wafer transfer system is actually ready to load or unload a cassette. 
     For various reasons, however, the process equipment  20  may not be in a suitable transfer mode when the AGV begins communicating. When transfer begins while the processing equipment  20  is not in a suitable mode, for example, if the door is not completely open, or the robot arm is not completely unfolded, the cassette might not be placed on the stage  21  at the preferred location A in FIG.  2 . 
     FIG. 2 shows that when a cassette is placed at the preferred location A on the stage  21 , the sensor S 1  can detect its presence. However, if the cassette is placed near the preferred location but not in the view of the sensor S 1 , the sensor S 1  may indicate to the process equipment controller that the stage is empty. In this case, the process equipment controller will not clear the stage upon receiving the load command from the host computer  30 . When the AGV  13  attempts to perform the load operation, the cassettes collide and the transfer operation fails. This faulty detection of cassette stage  21  occupancy is a main cause of failure for the transferring operations. In addition, cassettes may collide with a door that is not open or a robot arm that is not unfolded. For any of these reasons a cassette may be dropped during the failure of the transfer operation and wafers may be spilled. The current system is unable to check whether there are problems with the stage  21  occupancy or the internal mechanisms of the process equipment  20  prior to transferring wafer cassettes. 
     A need exists, therefore, for a wafer cassette transfer system that will not transfer cassettes if the process equipment is not in a suitable mode. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method for transferring a wafer cassette between an AGV and process equipment in a semiconductor fabrication facility. The method comprises transmitting a first signal from the AGV to the process equipment indicative of a cassette transfer request, and checking whether the process equipment is in a suitable mode for the transfer request. If the process equipment was not already in a suitable mode, the next step sets the process equipment to the suitable mode. Then transferring the cassette between the AGV and the process equipment occurs. 
     In another version of the invention, the method includes transmitting a second signal from the process equipment indicating authorization for the transfer of the cassette before the actual transferring occurs. In another version the checking step further comprises the step of sensing the position of the cassette on a cassette stage of the process equipment using a plurality of sensors. 
     In another aspect of the invention, a system for exchanging a cassette between an AGV and process equipment using the method comprises a host computer, an AGV controller, an AGV, process equipment, and a communications link between the AGV and the process equipment. The process equipment includes a cassette stage and a plurality of cassette sensors for detecting placement of the cassette at any position on the stage. 
     The invention provides several features which greatly reduce failure by ensuring that the process equipment is in a suitable mode before transferring occurs. In addition, by using a plurality of sensors, the occupancy of the cassette stage can be determined even if the cassette is not positioned at the preferred location on the stage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects, and advantages will be better understood from the following detailed description of the embodiments of the invention with reference to the drawings, in which: 
     FIG. 1 is a schematic diagram of a wafer cassette transfer system using a current embodiment; 
     FIG. 2 is a schematic diagram showing a loading/unloading cassette stage of the process equipment of FIG. 1; 
     FIG. 3A is a schematic diagram of a wafer cassette transfer system using an embodiment of the present invention; 
     FIG. 3B is a schematic diagram of a wafer cassette transfer system using another embodiment of the present invention; 
     FIG. 4 is a schematic diagram showing an embodiment of the loading/unloading cassette stage of the modified process equipment of FIG.  3 A and FIG. 3B according to the present invention; 
     FIG. 5 is a flowchart showing the method for transferring wafer cassettes according to an embodiment of the present invention; 
     FIG. 6 is a timing chart for loading operations showing communications across the communications link between the process equipment and the AGV according to an embodiment of the present invention; and, 
     FIG. 7 is a timing chart like FIG. 6 except for unloading operations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is directed to a method and system for transferring semiconductor wafer cassettes with process equipment in a suitable mode. 
     FIG. 3A illustrates a system for performing an embodiment of the method of the present invention. The wafer cassette transfer system has basically the same elements as previously described in FIG. 1, but the process equipment is modified to perform differently upon receiving communications from the AGV  13  through the communications link  15  and from the host computer  30 . In other later described embodiments, such as shown in FIG. 3B, the AGV  53  is also modified to perform differently, and the communications link  55  passes different information between the process equipment  40  and the modified AGV  53 . 
     FIG. 4 shows the loading/unloading cassette stage  41  of the modified process equipment  40  of the present invention. In one embodiment additional sensors are added to detect the position of cassettes on the stage  41 ; in the preferred embodiment two additional sensors S 2  and S 3  are added. The additional sensors may be beam type optical sensors, each having an emitter part and a detector part, for detecting the position of cassettes not placed in the preferred location A and therefore not sensed by the first sensor S 1 . In the preferred embodiment, the sensors S 2  and S 3  are located near the cassette stage  41  and need not be part of the cassette stage  41 . 
     The operation of the transfer system according to the method of the present invention will now be described. For simplicity, the embodiments of FIG.  3 A and FIG. 3B will be described simultaneously, where possible, referring to the AGV  13  in FIG.  3 A and the modified AGV  53  in FIG.  3 B. 
     As shown in the flow chart of FIG. 5, the method of the present invention begins with step S 10  in which the AGV  13 ,  53  initiates a communications link ( 15  in FIG. 3A and 55 in FIG. 3B) with the process equipment  40 , using for example, a PPIO link as in the preferred embodiment. The AGV  13 ,  53  then transmits a request signal to the process equipment  40  in step S 20 . In the preferred embodiment, the signal would designate a cassette stage  41  by a cassette stage number. In step S 30 , the process equipment  40  sets itself into a suitable mode, i.e., the process equipment  40  sets the designated cassette stage  41  and associated internal mechanisms, if any, into states appropriate for the type of transfer (load or unload) commanded by the host computer  30  or the AGV  13 ,  53 . Initially, this step S 30  might be performed before the AGV  13 ,  53  arrives, based on a command from the host computer  30 . In step S 40 , the process equipment  40  determines whether it is completely set in the suitable mode by checking the occupancy of the cassette stage  41  using the sensors provided, e.g., S 1 , S 2  and S 3  (FIG. 4) in the preferred embodiment, and by checking the internal mechanisms if any such checks are provided in the modified process equipment  40 . If the result of step S 40  is that the process equipment  40  is not in a suitable mode, step S 30  is repeated, followed by another execution of step S 40 . This cycle can be repeated until the process equipment  40  is found to be in the suitable transfer mode during step S 40 . 
     When it is determined during step S 40  that the process equipment  40  is in the suitable mode, then in step S 50  a wafer cassette is transferred between the AGV  13 ,  53  and the process equipment  40 . Execution of step S 50  may be made to occur after step S 40  determines the process equipment  40  is in a suitable mode, by delaying step S 50  until an authorization signal is received by the modified AGV  53  from the process equipment  40  over the modified communications link  55  as in the preferred embodiment. Alternatively, the delay required of the unmodified AGV  13  can be predetermined or provided by the host computer  30  based on communications with the process equipment  40 . 
     Finally, the method ends when the communications link  15 ,  55  is disestablished in steps S 60  and S 70 . In the preferred embodiment, step S 60  involves the removal of the signal designating the cassette stage number; and step S 70  involves the termination of the PPIO link. 
     The method for loading a wafer cassette onto the process equipment  40  according to the preferred embodiment in FIG. 3B will be described in further detail with reference to FIG.  6 . 
     Referring to FIG. 3B, the load operation begins with the host computer  30  issuing a load command to the controller  11  of the transfer equipment  50  and the controller (not shown) of the process equipment  40  according to the SECS. The controller  11  controls the movement of the modified AGV  53  through the radio link  12  to move to the process equipment  40  which is to be loaded. As a result the modified AGV  53  arrives at the process equipment  40  adjacent to its loading/unloading cassette stage  41 . 
     After arrival at the process equipment  40 , the modified AGV  53  begins to communicate with the process equipment  40  via the communications link  55 . This corresponds to step S 10  in FIG.  5 . 
     The communications across the communications link  55  are shown in FIG. 6 as time charts of various signals. In FIG. 6, signals originating in the modified AGV  53  in FIG. 3B are followed by the letter A in parentheses, and signals originating in the process equipment  40  in FIG. 3B are followed by the letter P in parentheses. At time ‘a’ the modified AGV transmits a Cs-No signal designating a cassette stage number. The Cs-No signal represents the cassette stage number as CS_ 0 , CS_ 1 , CS_ 2 , etc. Shortly thereafter, at time ‘b’, the modified AGV transmits a VALID status signal that is at a high level to indicate the status is ON. A VALID ON signal means that the cassette number transmitted by Cs-No is valid for the current process equipment  40  adjacent to the modified AGV  53 . The Cs-No signal and VALID ON signal correspond to step S 20  in FIG.  5 . 
     Meanwhile, the process equipment  40  has begun step S 30  to set itself into the ready for load mode. In the preferred embodiment, step S 30  is begun based on the initial communication from the host computer  30  or based on a subsequent return of control from step S 40  in FIG.  5 . During step S 30 , for example, if a door is included in the process equipment  40 , it is opened; if a robot arm is included it is unfolded. During step S 30  the sensors S 1 , S 2 , and S 3  determine whether a cassette is located anywhere on the cassette stage  41 , and, if so, the cassette is removed by the process equipment  40  under the control of the process equipment controller (not shown). 
     After receiving the VALID ON signal from the modified AGV  53 , the door sensors and arm sensors, if provided, of the process equipment  40  detect the state of the door and arm and provide the result to the process equipment controller (not shown). The cassette stage sensors S 1 , S 2 , and S 3  detect the occupancy state of the cassette stage  41  and provide the result to the process equipment controller. Based on these results the process equipment controller determines whether the process equipment  40  is in the suitable mode, i.e., the ready for load mode. If it is determined that the process equipment is not in the ready for load mode, the method returns control to step S 30 . For example, if a door is not open, or a robot arm is not unfolded, or the cassette stage  41  is not empty, control returns to step S 30 . Even if a cassette is not in the preferred location A in FIG. 4 of the stage  41 , the preferred embodiment of the present invention will detect that the stage  41  is not empty because of the additional sensors S 2  and S 3 . 
     While the process equipment  40  is not in the ready for load mode, the status signal L_REQ remains at a low signal level indicating L_REQ OFF. The next step (S 50 ) of the method to load the cassette, will not be performed while a L_REQ OFF signal is transmitted. Thus, transfer failures such as cassettes colliding with partly closed doors, or partly folded robot arms, or other cassettes already occupying a cassette stage  41  during the load operation are avoided. 
     When it is determined that the process equipment  40  is in the ready for load mode, the L_REQ status signal is changed to L_REQ ON at time ‘c’ by sending a high level signal. This corresponds to the end of step S 40  in FIG.  5 . 
     In the preferred embodiment, step S 50  in FIG. 5 begins with the modified AGV  53  changing the TR_REQ status signal from a low level to a high level to indicate TR-REQ ON at time ‘d’. This means that the modified AGV  53  is about to transfer the wafer cassette to the process equipment  40 . The process equipment  40  responds by changing a READY status signal from a low level signal indicating READY OFF to a high level signal indicating READY ON at time ‘e’. At time ‘f’, subsequent to time ‘e’, the physical transfer of the cassette from the modified AGV  53  to the process equipment  40  begins and the status signal BUSY is changed from a low level indicating BUSY OFF to a high level signal indicating BUSY ON. 
     Upon completion of the loading operation, the wafer cassette now placed on the cassette stage  40  is sensed by one or more of the sensors S 1 , S 2  and S 3 . At this time, time ‘g’, the process equipment  40  converts the high level signal indicating L_REQ ON to a low level signal indicating L_REQ OFF in the status signal L_REQ being transmitted to the modified AGV  53 . In response, the modified AGV  53  converts the status signal BUSY transmitted to the process equipment  40  from a high level indicating BUSY ON to a low level indicating BUSY OFF at time ‘h’. Also at time ‘h’, the status signal COMPT transmitted from the modified AGV  53  to the process equipment  40  is changed from a low level indicating COMPT OFF to a high level indicating COMPT ON to represent that the loading operation has completed. This corresponds to the completion of step S 50  in FIG.  5 . 
     In response to the COMPT ON signal, at time ‘i’ the process equipment  40  converts the status signal READY transmitted to the modified AGV  53  from a high level indicating READY ON to a low level indicating READY OFF. 
     In response to the READY OFF signal, at time ‘j’, the modified AGV  53  converts the BUSY, VALID, and Cs-No signals transmitted to the process equipment  40  from high levels to low levels indicating BUSY OFF, VALID OFF, and Cs-No OFF, respectively. The change to Cs-No OFF at time ‘j’ corresponds to removing the designation of the cassette stage number in step S 60  in FIG.  5 . 
     When the designation of the cassette stage number is removed, the modified AGV  53  terminates the PPIO link with the process equipment  40  corresponding to step S 70  in FIG.  5 . In the event that there is not another subsequent transfer operation with the current process equipment  40 , the modified AGV  53  moves away from the process equipment  40 . 
     The method for unloading a wafer cassette from the process equipment  40  according to the preferred embodiment will be described in further detail with reference to FIG.  3 B and FIG.  7 . The method is the same and the details are similar to that described immediately above in detail for the load operation, except for the use of a different status signal, U_REQ instead of L_REQ, the suitable mode being ready for unload rather than ready for load. 
     As described above and shown in FIG. 3B, the modified AGV  53  arrives at the process equipment  40  adjacent to its loading/unloading cassette stage  41 . After arrival at the process equipment  40 , the modified AGV  53  begins to communicate with the process equipment  40  via the communications link  55 . This corresponds to step S 10 . 
     The communications across the communications link  55  are shown in FIG. 7 as time charts of various signals during the unload operation. In FIG. 7, signals originating in the modified AGV  53  are followed by the letter A in parentheses, and signals originating in the process equipment  40  are followed by the letter P in parentheses. The modified AGV transmits a Cs-No signal designating a cassette stage number at time ‘a’. The Cs-No signal represents the cassette stage number as CS_ 0 , CS_ 1 , CS_ 2 , etc. The modified AGV transmits a VALID status signal at time ‘b’ that is at a high level to indicate the status is ON. A VALID ON signal means that the cassette number transmitted by Cs-No is valid for the current process equipment  40  adjacent to the modified AGV  53 . This pair of signals correspond to step S 20 . 
     Meanwhile, the process equipment  40  has begun step S 30  to set itself into the ready for unload mode. In the preferred embodiment, step S 30  is begun based on the initial communication from the host computer  30  or based on a subsequent return of control from step S 40 . 
     During step S 30 , for example, if a door is included, it is opened; if a robot arm is included it is unfolded. During step S 30 , the sensors S 1 , S 2 , and S 3  determine whether a cassette is located anywhere on the cassette stage  41 ; and, if not, a cassette is placed on the stage  41  by the process equipment  40  under the control of the process equipment controller (not shown). 
     After receiving the VALID ON signal from the modified AGV  53  the door sensors and arm sensors, if any, of the process equipment  40  detect the state of the door and arm and provide the result to the process equipment controller (not shown). The cassette stage sensors S 1 , S 2 , and S 3  detect the occupancy state of the cassette stage  41  and provide the result to the process equipment controller. Based on these results the process equipment controller determines whether the process equipment  40  is in the suitable mode, i.e., the ready for unload mode. If it is determined that the process equipment is not in the ready for unload mode, the method returns control to step S 30 . For example, if a door is not open, or a robot arm is not unfolded, or the cassette stage  41  is not occupied, control returns to step S 30 . Even if a cassette is not in the preferred location A of the stage  41 , the preferred embodiment of the present invention will detect that the stage  41  is occupied because of the additional sensors S 2  and S 3 . 
     While the process equipment  40  is not in the ready for unload mode, the status signal U_REQ remains at a low signal level indicating U_REQ OFF. The next step (S 50 ) to unload the cassette, will not be performed while a U_REQ OFF signal is transmitted. Thus, transfer failures such as cassettes colliding with partly closed doors, or partly folded robot arms, or having no cassette to unload on a cassette stage  41  during the unload operation are avoided. 
     When it is determined that the process equipment  40  is in the ready for unload mode, the U_REQ status signal is changed to U_REQ ON at time ‘k’ by sending a high level signal. This corresponds to the end of step S 40 . 
     Step S 50  in the preferred embodiment begins with the modified AGV  53  changing the TR_REQ status signal from a low level to a high level to indicate TR_REQ ON at time ‘d’. This means that the modified AGV  53  is about to transfer the wafer cassette from the process equipment  40 . The process equipment  40  responds by changing a READY status signal from a low level signal indicating READY OFF to a high level signal indicating READY ON at time ‘e’. At time ‘f’, subsequent to time ‘e’, the physical transfer of the cassette to the modified AGV  53  from the process equipment  40  begins and the status signal BUSY is changed from a low level indicating BUSY OFF to a high level signal indicating BUSY ON. 
     Upon completion of the unloading operation, the absence of a wafer cassette on the cassette stage  40  is sensed by all of the sensors S 1 , S 2  and S 3 . At this time, time ‘m’, the process equipment  40  converts the high level signal indicating U_REQ ON to a low level signal indicating U_REQ OFF in the status signal U_REQ being transmitted to the modified AGV  53 . In response, the modified AGV  53  converts the status signal BUSY transmitted to the process equipment  40  from a high level indicating BUSY ON to a low level indicating BUSY OFF at time ‘h’. Also at time ‘h’, the status signal COMPT transmitted from the modified AGV  53  to the process equipment  40  is changed from a low level indicating COMPT OFF to a high level indicating COMPT ON to represent that the unloading operation has completed. This corresponds to the completion of step S 50 . 
     In response to the COMPT ON signal, at time ‘i’ the process equipment  40  converts the status signal READY transmitted to the modified AGV  53  from a high level indicating READY ON to a low level indicating READY OFF. 
     In response to the READY OFF signal, at time ‘j’, the modified AGV  53  converts the BUSY, VALID, and Cs-No signals transmitted to the process equipment  40  from high levels to low levels indicating BUSY OFF, VALID OFF, and Cs-No OFF, respectively. The change to Cs-No OFF at time ‘j’ corresponds to removing the designation of the cassette stage number in step S 60 . 
     When the designation of the cassette stage number is removed, the modified AGV  53  terminates the PPIO link with the process equipment  40  corresponding to step S 70 . In the event that there is not another subsequent transfer operation with the current process equipment  40 , the modified AGV  53  moves away from the process equipment  40 . 
     As described above, the method of the present invention prevents many failures of the wafer cassette transfer process by checking that the process equipment  40  is in a suitable mode before performing the transfer. A system embodiment of the present invention that permits the performance of the preferred embodiment of the method requires that additional sensors S 2  and S 3  be placed near the loading/unloading cassette stage  41  of the process equipment  40 . The additional sensors S 2  and S 3  are used to detect the position of a cassette that is not located at the preferred location A in FIG. 4 on the stage  41 . 
     It is understood that many modifications are contemplated within the practice of the present invention. Specifically, the placement of sensors to detect the positions of cassettes at locations other than the preferred location can be varied widely. Also, the number of status signals passed back and forth between the AGV and the process equipment can be varied down from the eight described in the preferred embodiment. In addition, communications links other than the PPIO link can readily be employed. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the system and method of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.