Abstract:
A system is disclosed for ensuring that the pod door is firmly and securely retained on the port door of a process tool as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool. The system includes a latch key protruding outwardly from the outer surface of the port door. The latch key is provided to mate within a slot of a door latch assembly within the pod door. Once the latch key is properly seated within the slot, the latch key is rotated by mechanisms within the port door to decouple the pod door from the pod shell. Such rotation at the same time couples the pod door to the port door. According to the present invention, while the latch key rotates, it simultaneously moves in the rearward direction (i.e., back toward the port door) to thereby pull the pod door into a tight engagement with the port door.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the transfer of workpieces such as semiconductor wafers from a storage and transport pod to a process tool, and in particular to a system for ensuring that the pod door is firmly and securely retained on the port door as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool. 
     2. Description of Related Art 
     A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment. 
     A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled “SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,” by Mihir Parikh and Uhrich Kaempf,  Solid State Technology,  July 1984, pp. 111-115. 
     Systems of the above type are concerned with particle sizes which range from below 0.02 microns (μm) to above 200 μm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half μm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 μm substantially interfere with μm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 μm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest. 
     SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called “bottom opening” pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide “front opening” pods, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell. For both front opening and bottom opening pods, a pod door includes a front surface which is included as part of the sealed pod environment, and a rear surface which is exposed to the environment of the wafer fab. 
     In order to transfer wafers between a SMIF pod and a process tool within a wafer fab, a pod is typically loaded either manually or automatedly onto a load port on a front of the tool. The process tool includes an access port which, in the absence of a pod, is covered by a port door which includes a front surface exposed to the wafer fab environment and a rear surface which is part of the sealed environment within the process tool. The SMIF pod is seated on the load port so that the pod door and port door lie adjacent to each other. Registration pins are provided on the port door that mate with grooves in the pod door to assure a proper alignment of the pod door with respect to the port door. 
     Once the pod is positioned on the load port, mechanisms within the port door unlatch the pod door from the pod shell and move the pod door and port door together into the process tool where the doors are then stowed away from the wafer transfer path. The pod shell remains in proximity to the interface port so as to maintain a clean environment including the interior of the process tool and the pod shell around the wafers. A wafer handling robot within the process tool may thereafter access particular wafers supported in the pod for transfer between the pod and the process tool. 
     It is extremely important to provide a clean, low particulate and contaminant environment around the exposed wafers within the process tool. While the air within wafer fabs is typically filtered to some degree, the environment surrounding the process tools and SMIF pods include relatively high levels of particulates and contaminants as compared to within the pods and tools. As such, significant steps are taken to isolate SMIF pod and process tool interiors from the surrounding environment within the fab. 
     As explained above, the pod door and port door, even though having surfaces exposed to the environment of the wafer fab, are typically brought into the interior of the process tool in preparation for wafer transfer between the pod and the tool. In order to prevent the particulates and contaminants on the exposed door surfaces from contaminating the interior of the process tool, it is known to hold the exposed pod and port door surfaces against each other when bringing the pod and port doors into the process tools and while the doors are positioned therein. Such contact may trap particulates and/or contaminants between the exposed surfaces to thereby prevent the transfer of the particulates and/or contaminants into the process tool. 
     Coupling mechanisms are known for coupling the pod door to the port door as the pod door is removed from the pod and stowed in the process tool. However, without additional restraints between the pod and port doors, it is possible that the pod door will vibrate on the port door, or that the pod door will tilt or otherwise move with respect to the port door. Any such vibration or movement may result in particulates and/or contaminants dislodging from the pod and/or port door surfaces and settling in the process tool. 
     Prior art attempts have been made to hold the pod door firmly against the port door while the doors are coupled together and stowed in the process tool. One such system is disclosed in U.S. Pat. No. 5,772,386, entitled “Loading and Unloading Station for Semiconductor Processing Installations”, which patent is assigned to Jenoptik A. G. As set forth therein, the port door may include a pair of suction cups connected to a vacuum source. When the pod door is coupled to the port door, the suction cups engage a surface of the pod door, and the vacuum source creates suction within the cups to hold the pod door to the port door. In addition to the fact that particulates and contaminants may still escape from between the doors into the process tool, there is a further disadvantage to the disclosed system in that the vacuum source may fail or that the suction between the pod and port door may otherwise be lost. In such an instance, the pod door would potentially be able to vibrate or move around with respect to the port door as explained above. Moreover, this type of system requires a fab to provide a vacuum source as an additional utility to the process tool. Not only does this increase the cost and complexity of the tool design, but the control system must also include routines for monitoring the vacuum source to ensure proper operation. 
     SUMMARY OF THE INVENTION 
     It is therefore an advantage of the present invention to minimize the amount of particulates and contaminants that may dislodge from between the pod and port doors in the process tool. 
     It is another advantage of the present invention to provide a tight seal between the port and pod doors when the doors are latched together to prevent the escape of contaminants and/or particulates from between their juxtaposed surfaces. 
     It is a further advantage of the present invention to establish a tight seal between the port and pod doors entirely by mechanisms within the port and pod doors, without requiring additional monitors and/or utilities such as a vacuum source for the process tool. 
     It is a still further advantage of the present invention to provide a system for firmly holding the port and pod doors together, which system is manually adjustable to accommodate for varying thicknesses of the port and pod doors. 
     It is another advantage of the present invention to provide a self-adjusting system in addition to or instead of the above-described manual adjustment system for accommodating variations in port and pod door thicknesses and for providing for engineering tolerances. 
     It is another advantage of the present invention to provide a system capable of providing a tighter coupling of a pod door to a port door than with conventional suction systems. 
     These and other advantages are provided by the present invention, which in preferred embodiments relates to mechanisms for pulling the pod door tightly against the port door as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool. In a preferred embodiment, the mechanism includes a latch key protruding outwardly from the outer surface of the port door. The latch key is provided to mate within a slot of a door latch assembly within the pod door. Once the latch key is properly seated within the slot, the latch key is rotated by mechanisms within the port door to decouple the pod door from the pod shell. Such rotation at the same time couples the pod door to the port door. According to preferred embodiments of the present invention, while the latch key rotates, it simultaneously moves in the rearward direction (i. e., back toward the port door) to thereby pull the pod door into a tight engagement with the port door. 
     In order to provide rearward translation of the latch key upon rotation, in a preferred embodiment, the latch key is affixed to a shaft including a rear threaded section. The threaded section is received within a threaded nut mounted within the port door so that the rotation of the latch key also moves the latch key rearward with respect to the nut. 
     The nut may be affixed within the port door by screws fitting within adjustment slots provided through the nut. While the latch key is in a stationary position, loosening of the screws from within the adjustment slots allows rotation of the nut to the extent of the slots, which in turn translates the latch key forward or rearward with respect to the outer surface of the port door. This adjustment mechanism allows the height of the latch key in front of the port door surface to be adjusted to accommodate variations in pod door thicknesses. Moreover, as a given rotation of the nut will result in a relatively small translation of the latch key, the nut adjustment assembly is capable of providing fine adjustment of the position of the latch key past the surface of the port door. The adjustment may be made more or less fine by decreasing or increasing, respectively, the pitch of the threads. Altering the thread pitch will also vary the translation of the latch key for a given rotation of the latch key. 
     In alternative embodiments of the present invention, the nut may be mounted on a spring loaded plate which allows self-adjustment of the axial position of the latch key. Such an alternative embodiment accommodates variations in port and pod door thicknesses, and also provides for engineering tolerances within the port and pod doors. 
     The latch key and slot that receives the latch key preferably include smooth surfaces to minimize particulate generation as the latch key rotates in the slot. However, in the event particulates are generated, they are trapped within the pod door. In a further alternative embodiment, the latch key may be provided with rollers which lie in contact with the walls of the slot as the latch key rotates to further prevent particulate generation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described with reference to the figures, in which: 
     FIG. 1 is a perspective view of a front opening SMIF pod located next to the port door of a process tool; 
     FIG. 2 is an enlarged perspective view of a portion of the outer surface of a port door including a latch key protruding outwardly therefrom; 
     FIG. 2A is an alternative latch key configuration to that shown in FIG. 2 including rollers; 
     FIG. 3 is a front view of the interior of a port door including mechanisms for rotating the latch key; 
     FIG. 4 is an exploded perspective view illustrating the latch key and mounting components according to the present invention for allowing rotation and translation of the latch key; 
     FIG. 5 is a perspective view of the assemblies mechanism in the port door for supporting, rotating and translating the latch key; 
     FIG. 6 is a perspective view of a latch key and mounting components for allowing rotation and translation of the latch key according to an alternative embodiment of the present invention; 
     FIG. 7A is a top view of a latch key and mounting components for allowing rotation and translation of the latch key according to a further alternative embodiment; 
     FIG. 7B is a side view of the embodiment of the present invention shown in FIG. 7A; and 
     FIG. 7C is a top view of the embodiment of the present invention shown in FIG. 7A in a retracted position. 
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described with reference to FIGS. 1-7C which in preferred embodiments relate in general to mechanisms for decoupling a pod door from a pod and for tightly coupling the pod door to the port door as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool. While a preferred embodiment of the present invention operates in conjunction with a SMIF pod, it is understood that the present invention may operate with any of various containers. This includes 200 mm and 300 mm SMIF pods, bottom opening and front opening SMIF pods, and pods/boxes that do not operate according to SMIF technology. Additionally, the present invention may operate with containers carrying any of various workpieces, including wafers, reticles, and flat panel displays. The structure according to the present invention complies with and allows compliance with all applicable SEMI standards. 
     FIG. 1 is a perspective view of a 300 mm front opening SMIF pod  20  including a pod door  22  mating with a pod shell  24  to define a sealed environment for one or more workpieces located therein. (The rear of the pod door  20  would ordinarily be facing the port door as the pod is loaded on the port. It is shown otherwise in FIG. 1 for clarity.) While pod  20  is illustrated as a 300 mm front opening pod, as previously indicated, the size and type of the pod are not critical to the present invention. In order to transfer the workpieces between the pod  20  and a process tool  28 , the pod is loaded onto a load port  25  adjacent a port door  26  on a front of the process tool. The type of process carried out within tool  28  is not critical to the present invention, and may be any of various testing, monitoring, and/or processing operations. 
     Referring now to FIGS. 1 and 2, a front surface  30  of the port door  26  faces a rear surface  31  of the pod door  22 , and includes a pair of latch keys  32  for being received in a corresponding pair of slots  33  of a door latching assembly mounted within pod door  22 . An example of a door latch assembly within a pod door adapted to receive and operate with latch key  32  is disclosed in U.S. Pat. No. 4,995,430 entitled “Sealable Transportable Container Having Improved Latch Mechanism”, to Bonora et al., which patent is assigned to the owner of the present invention, and which patent is incorporated by reference herein in its entirety. In order to latch the pod door to the port door, the pod door  22  is seated adjacent the port door  26  so that the vertically oriented latch keys are received within the vertically oriented slots  33 . 
     In addition to decoupling the pod door from the pod shell, rotation of the latch keys  32  will also lock the keys onto their respective slots  33 , thus coupling the pod door to the port door. As explained in greater detail below, an alternative latch key  32  is shown in FIG. 2A including rollers  35  mounted on a pin  37  of the key. A preferred embodiment of the present invention includes two latch key  32  and slot  33  pairs, each of which pairs are structurally and operationally identical to each other. As such, the following description may at times discuss only one of the latch keys and/or slots. However, it is understood that the description of the latch keys and slots, and the components associated therewith, applies equally to both of the latch keys and slots. However, in alternative embodiments of the invention, the pod door may be coupled to the port door by a single latch key and slot pair, or more than two latch key and slot pairs. Moreover, it is understood that where there is more than one latch key/slot pair, the respective pairs need not be identical to each other in alternative embodiments of the invention. 
     While a preferred embodiment of the door latch assembly in the pod door has been described above, it is understood that the mechanisms in the pod door for coupling/uncoupling the pod door to the pod shell are not critical to the present invention and may vary significantly in alternative embodiments. 
     In a preferred embodiment, the latch keys  32  perform duel functions. They tightly couple/uncouple the port and pod doors, and they also actuate coupling/uncoupling of the pod door and pod shell. However, it is understood that the latch keys  32  may perform only one or the other of these functions in alternative embodiments. For example, in one such alternative embodiment, latch keys  32  provide no coupling/uncoupling functions between the pod door and pod shell. In such an embodiment, the latch key  32  would merely latch and unlatch the pod door to and from the port door in accordance with the principles of the present invention. An example of a system where the latch keys provide no coupling/uncoupling functions between the pod door and pod shell is disclosed for example in U.S. patent application Ser. No. 08/998,115 entitled “SMIF Pod Door and Port Door Removal and Return System”, to Bonora et al., which application is assigned to the owner of the present invention, and which application is incorporated by reference in its entirety herein. Additionally, in a further alternative embodiment, the latch keys may only be provided to actuate coupling and uncoupling of the pod door to the pod shell. In such an embodiment, as discussed in greater detail below, mechanisms other than the latch keys  32  may be provided for tightly coupling and uncoupling the pod door to the port door. 
     With respect now to preferred embodiments of the invention, the structures in the port door for actuating the latch keys  32  will be described with reference to the rear view of FIG. 3, and the perspective views of FIGS. 4 and 5. The latch keys  32  are affixed to respective latch key mounting assemblies  34 , explained in greater detail below. An actuator  36  is fixedly mounted to each of the latch keys  32  (as best seen in FIG. 4) which actuators  36  are connected to each other by a translating rod  38 . In a preferred embodiment, once a pod is seated adjacent a port door  26  (as indicated for example by a pod-at-port sensor), a motor  40  drives a pair of pulleys  42  and  44  attached to each other via a timing belt  46 . Pulley  44  is in turn attached to a lead screw  48  having a carriage  50  mounted thereon, which carriage moves back and forth along the lead screw upon the screw rotation. The carriage  50  is in turn connected to the translating rod  38  affixed to the actuators  36 . Thus, rotation of the motor will cause translation of the rod  38  and a pivoting of the actuators  36  to thereby rotate the latch keys  32 . As would be appreciated by those of skill in the art, various mechanisms and linkages may be substituted for those described above for transferring torque from the motor to the actuators  36  to thereby rotate the latch keys  32 . 
     Referring specifically now to FIGS. 4 and 5, the latch key mounting assembly includes a stationarily mounted bearing support block  52  including a bearing  54  fitting within a hole  56  formed in the port door. The latch key  32  includes a shaft  58  extending rearwardly therefrom, which shaft is rotatably supported within bearing  54  of the bearing support block  52 . As previously indicated, an actuator  36  is also mounted along the shaft  58  and, for example, secured thereto by a pin  60 . Thus, the latch key  32  is constrained to rotate with the actuator  36 . 
     The latch key mounting assembly  34  further includes a nut mounting block  62  which is stationarily mounted to the bearing support block  52 . As the actuator  36  is fixedly mounted to the latch key shaft  58 , a space may be provided between the nut mounting block  62  and the bearing support block  52  to allow slight translation of the actuator along the axis of rotation of the latch key as explained hereinafter. 
     The latch key mounting assembly  34  further includes an axial adjustment nut  64  which is adjustably mounted to the nut mounting block  62  by a pair of screws  66 . In particular, the screws  66  fit through respective arcuately shaped slots  68  provided axially through the adjustment nut  64  and into one of two alternate pairs of countersunk holes  63  in the nut mounting block. Upon loosening of screws  66 , the adjustment nut  64  may be rotated to the extent allowed by the slots  68 . The two alternate pairs of countersunk hole  63  are provided (as opposed to one such pair) to allow adjustment of the adjustment nut around 360°. In particular, with the screws  66  provided in a first alternate pair of holes  63 , the nut may be adjusted to a certain rotational extent (defined by the arc length of the slots  68 ). However, if the screws  66  are thereafter removed and inserted in the second alternate pair of holes  63 , further rotational adjustment may then be obtained. 
     A rear section of latch key shaft  58  includes threads  70 . The shaft  58  fits through the bearing  54 , an opening in the actuator  36  and nut mounting block  62 , and is received within the central opening of the adjustment nut  64 . The central opening in the adjustment nut includes threads that mate with the threads  70 . Thus, as in a common nut and bolt arrangement, any relative rotation between the adjustment nut  64  and the latch key  32  will also result in translation of the latch key with respect to the adjustment nut and the port door in general. 
     In operation, when a pod door is initially seated adjacent the port door and the latch key is received within the door latch assembly slot  33 , motor  40  will rotate the actuators  36 , which in turn rotate the respective latch keys  32  to lock the pod door onto the port door. Additionally, according to the present invention, rotation of the latch key threads  70  within the threaded central opening in the adjustment nut  64  causes the latch key  32  to move rearwardly toward the port door as it rotates. The latch key  32  will engage the rear walls of the slot  33  as it translates rearward, thereby pulling the pod door into tight engagement against the port door. 
     In a preferred embodiment of the present invention, the actuator  36  may rotate the pod door latch key approximately 90°. The pitch of threads  70  may be approximately 100 to 150 mils, so that a 90° rotation of the latch key results in an approximate 25 to 37 mils translation of the latch key back toward the surface of the port door. It is understood that the pitch of threads  70  may be lesser or greater than 100 to 150 mils in alternative embodiments of the present invention. 
     On occasion, thicknesses of the pod and/or port doors may vary, or it may otherwise be desirable to slightly adjust the distance by which the latch key protrudes past the front surface of the port door. In order to accomplish this, screws  66  are loosened to allow rotation of the axial adjustment nut  64 . During such rotation, the actuators  36  prevent rotation of the latch keys, so that rotation of the adjustment nut  64  will axially translate the latch key to protrude a greater or lesser extent past the front surface of the port door. The degree to which the axial position of the latch keys may be adjusted can be varied by increasing or decreasing the arcuate lengths of the slots  68  in the axial adjustment nut  64  and/or by varying the pitch of threads  70 . As a given rotation between the nut and latch key will result in a relatively small translation of the latch key, the nut adjustment assembly is capable of providing a fine adjustment of the position of the latch key past the surface of the port door. The adjustment may be made more or less fine by decreasing or increasing, respectively, the pitch of the threads  70 . In alternative embodiments, it is understood that the slots  68  may be replaced by screw holes to omit the above-described adjustment feature. 
     An alternative embodiment of the present invention is shown in the exploded perspective view of FIG.  6 . FIG. 6 is identical to the embodiment disclosed with respect to FIGS. 4 and 5 with the exception that the nut mounting block  62  of the above-described embodiments is omitted and is instead replaced generally by a guide pin block  72  and a nut mating plate  74 . In particular, the guide pin block  72  is affixed to the bearing support block  52  so as to be stationarily mounted within the port door. The guide pin block  72  includes at least two rearwardly extending guide pins  76  which guide pins include springs  78  circumjacent thereabout. The nut mating plate  74  includes holes  80  corresponding in number and position to guide pins  76 . Holes  80  have a slightly larger diameter than the guide pins  76 , but a smaller diameter than that of springs  78 . The nut mating plate  74  is affixed to the guide pin block  72  by retaining rings  82 , which are fastened to a rear section of the guide pins after the guide pins  76  have been fit through the holes  80  in the nut mating plate  74 . In such an arrangement, the nut mating plate  74  is capable of moving forward toward the guide pin block  72  against the biasing force of the springs  78  mounted around the guide pins  76 . 
     The axial adjustment nut  64  is affixed by screws  66  to the nut mating plate. As above, the adjustment nut  64  may be made adjustable as a result of the screws  66  fitting within the arcuate slots  68 . It is understood that the slots  68  may be omitted in this embodiment and replaced by screw holes to prevent rotation of, and manual adjustment by, the nut  64 . 
     In the embodiment of FIG. 6, the shaft  58  extends rearward through the bearing  54 , the opening in the actuator  36 , and through central openings in the guide pin block and nut mating plate so that the threads are received within the central opening of the adjustment nut  64  as described above. According to this embodiment, in addition to any manual adjustment of the extent to which the latch key protrudes past the surface of the port door, the guide pin block and nut mating plate together provide self-adjustment of the latch key mounting assembly to accommodate pod and port doors of varying thicknesses, and to provide for engineering tolerances. In particular, in the event a pod door is sufficiently tight against the port door to satisfy objectives of the present invention before the latch key  32  has finished its rotational stroke, instead of further translation of the latch key rearward, the continued rotation of the latch key will instead result in a translation of the adjustment nut  64  and nut mating plate  74  forward toward the guide pin block  72  by compressing the springs  78 . 
     It is understood that adjustment of the angular position of the adjustment nut  64  may in part control the point at which the nut mating plate  74  begins to move toward the guide pin block against the force of springs  78 . Additionally, it is understood that the desired compressive force between the port and pod doors according to the present invention may be varied by varying the spring constant and/or degree of preloading of springs  78 . 
     The latch key  32  and slot  33  that receives the latch key preferably include smooth surfaces to minimize particulate generation as the latch key rotates in the slot. Even if particulates are generated, they are trapped within the pod door and would not effect the environment within the process tool. In an alternative embodiment of the present invention shown in FIG. 2A, the latch key may be provided with rollers  35  mounted on a pin  37  through the front end of the shaft  58 . The rollers lie in contact with walls of the slot  37  as the latch key translates rearward. The rollers allow the latch key to rotate against the slot  37  wall without generating particulates, even upon a large compressive force between the latch key rollers and the slot walls. 
     Although a preferred embodiment of the latch key mounting assembly according to the present invention utilizes the rotation of the latch key to also bring about translation of the latch key, it is understood that the latch key may be translated independently of its rotation. Alternative mechanical systems may be employed for causing the desired translation of latch key  32  and tight engagement between the pod door and port door in alternative embodiments of the present invention. One such alternative embodiment is shown in FIGS. 7A-7C. Referring first to FIGS. 7A and 7B, there is shown a top view of the latch key  32 , and the shaft  58  extending rearwardly therefrom and extending through the port door  26 , bearing support block  52 , and the opening in the actuator  36 . This embodiment farther includes a U-shaped bracket  82  translatably mounted within the port door, a washer  84  fixedly attached to the end of shaft  58 , and a helical spring  86  wrapped around the shaft  58  and compressed between a front wall  88  of the bracket  82  and the washer  84 . 
     According to the alternative embodiment shown in FIGS. 7A-7C, in a relaxed state, the washer  84  is biased rearwardly by spring  86  and abuts against a rear wall  90  of the bracket  82 . In one embodiment, the bracket  82  is initially translated forward (i.e., toward the port door  26 ) by a known translation mechanism. For example, the bracket  82  may be affixed to and driven by a lead screw or solenoid. This forward translation allows the latch key  32  to seat within the slot in the door latch assembly in the pod door. In alternative embodiments, the latch key may initially be located far enough in front of the port door so that no initial forward translation of the translating bracket  82  is necessary. After or while the actuator  36  rotates latch key  32  to couple the pod door to the port door as described above, the translating bracket may translate rearwardly. Upon such rearward translation, the front wall  88  of bracket  82  will exert a force on spring  86 , which in turn exerts a force on washer  84  to move the shaft  58  and latch key  32  rearward, thus pulling the pod door more tightly against the port door. 
     At some point during the rearward translation of the bracket  82 , the pod door  22  will be held sufficiently tight against the port door  26  to accomplish objectives of the present invention. At this point, the force opposing rearward translation of the latch key  32 , shaft  58 , and washer  84  will overcome the force of spring  86 , at which point spring  86  will begin to compress as shown in FIG.  7 C. At some predetermined point prior to spring  86  becoming completely compressed, the rearward translation of bracket  82  will cease. Thus, according to this alternative embodiment, spring  86  will act to hold the pod door tightly against the port door, and the mechanism will be self-adjusting to port doors and pod doors of varying thicknesses and tolerances. As in the embodiment of the invention shown in FIG. 6, the desired compressive force between the port and pod doors may be varied by varying the spring constant and/or degree of preloading of spring  86 . 
     As would be appreciated by those of skill in the art, other configurations where translation of the latch key is accomplished independently of its rotation are possible. Another such alternative configuration is similar to that shown in FIGS. 7A through 7C, but the spring  86  and U-shaped bracket  82  may be omitted. In this embodiment, generally, the shaft  58  and latch key would be affixed to a driver such as a lead screw or solenoid which would translate the latch key rearward during or after coupling of the pod door to the port door to provide a tight coupling between the doors. The shaft  58  may, for example, be mounted in a thrust bearing which is in turn mounted for translation to the driver. The thrust bearing would allow the shaft  58  and latch key  32  to rotate, while also exerting an axial load on the shaft to translate the shaft and latch key. According to this embodiment, during or after coupling of the pod door to the port door, the driver would drive the shaft  58  and latch key  32  rearward until a tight engagement between the pod and port doors is established. 
     In preferred embodiments of the present invention described above, in addition to actuating the pod unlatch assembly and coupling the pod door to the port door, the latch keys  32  establish a tight contact between the pod door and the port door. However, it is understood that mechanisms other than the latch key may accomplish the objective of pulling the pod and port doors into tight engagement. For example, U.S. Pat. No. 4,534,389, entitled “Interlocking Door Latch For Dockable Interface For Integrated Circuit Processing”, discloses a spring loaded latch and release cable (FIG. 5 of that Patent) for holding a pod door against the port. 
     In a further alternative embodiment (not shown), it is contemplated that the port door include one or more magnets mounted in its front surface, which magnets align with a corresponding number of magnets on the rear surface of the pod door. Upon loading of the pod door onto the load port, the N-S poles of the port magnets align with the S-N poles, respectively, of the pod magnets so that the pod door is attracted into firm engagement with the port door. The magnets in the port (or, alternatively in the pod) may be rotationally supported in the port so as to be able to rotate about an axis perpendicular to the surfaces of the port and pod doors. When the pod door is to be returned to the pod, the magnets are rotated so that the N-S poles of the port magnets align with the N-S poles, respectively, of the pod magnets. In this position, juxtaposed magnets will repel each other, and the pod door may be returned to the pod. 
     Although the invention has been described in detail herein, it should be understood that the invention is not limited to the embodiments herein disclosed. Various changes, substitutions and modifications may be made thereto by those skilled in the art without departing from the spirit or scope of the invention as described and defined by the appended claims.