TECHNICAL FIELD 
     The present invention broadly relates to semiconductor wafer processing equipment, and deals more particularly with an improved pod employed to protectively enclose and transfer a cassette containing a plurality of semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     The high level of automation used in fabricating semiconductor devices relies on sophisticated handling and transport equipment for moving semiconductor wafers between various processing stations. Most handling and transport operations are conducted under automatic control using a programmed computer which issues control signals for operating the equipment with little or no intervention by an operator. In many systems, standard mechanical interface (SMIF) pods are used to transport batches of wafers that are stored in cassettes. These pods include a base upon which the cassettes rest, and a cover removably secured to the base and completely enclosing the cassette. The cover protectively surrounds the cassette, and thus the wafers, from the surrounding environment which may contain airborne, contamination particles. SMIF pods are most often used to transport cassettes from one clean room environment to another, where during the transport movement, the wafers, if not covered, are exposed to the contaminating environment. 
     After a pod has reached the vicinity of a processing station within a protected, clean room environment, it is necessary to remove the cover so that automated wafer transfer robots can gain access the individual wafers held in the cassette. The covers are held on the pods by various types of latching mechanisms which are automatically actuated to latch and unlatch the cover by means of actuating controls positioned at each processing station. Thus, when a pod reaches a processing station, control mechanisms engage the latch mechanism on the pod to unlatch the cover, following which either an operator or a robotic mechanism removes the cover to expose the cassette. Following processing of a batch of wafers in the cassette, the cover is reinstalled on the pod base, either manually or robotically, after which the control mechanism is actuated to latch the cover on the base before the pod leaves the processing station. 
     In spite of the fact that positive latch mechanisms are employed to lock the cover on the pod base, occasions arise when, for a variety of reasons, the latch fails to lock the cover on the pod base. This may occur, for example, when a foreign article becomes lodged between the cover and the base or where the cassette becomes tilted on the base, thus interfering with proper seating of the cover. In other cases, the control mechanism for actuating the latch may malfunction. In many cases, failure of the latch mechanism to lock the cover on the pod base goes undetected by process operators. As a result, it is possible that contaminants may pass between the pod base and cover when the pod leaves the clean room environment thereby resulting in possible contamination of the wafers. 
     Accordingly, there is a clear need in the art for an improved pod construction which provides a positive indication of when the cover is properly locked down on the pod base. The present invention is directed toward satisfying this need in the art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a pod for transporting a cassette containing a plurality of semiconductor wafers. The pod includes a base for supporting the cassette thereon, and a cover removably carried on the base for protectively covering the cassette. Latch means are carried on the base for latching the cover on the base. The latch means is actuatable from a closed, latched position in which the cover is locked down on the base, to a released, open condition allowing the cover to be removed from the base. Means are provided on the base for providing an operator with a positive, visual indication that the latching means has properly locked down the cover on the base. The indicator means includes an electrical power source, preferably a battery, carried on the base, an indicator light such as an LED, and an electrical switch which is actuated by the latch means. The latch means include a pair of slidably latch members on the base which engage and lock the cover, as well as a rotatable drive member on the base which is powered by a separate control mechanism at a processing station. The drive member preferably comprises a rotatable plate having a pair of drive pins on the outer periphery thereof. The latch members each include a slot therein for receiving a corresponding one of the drive pins. The drive pins translate the rotational motion of the drive member to linear motion, causing the latch members to slide in either of two directions respectively corresponding to an open position in which the cover may be removed from the base, and a closed, locked position in which the cover is secured on the base. 
     Both the drive member and the latch members are formed of electrically nonconductive material, while the drive pins are electrically conductive. The electrical switches used to switch power to the indicator lights are carried on the latch members and are switched by movement of the drive pins which act as electrical contacts. 
     It is therefore a primary object of the present invention to provide a semiconductor wafer cassette carrying pod that eliminates the possibility of wafer breakage or damage as the result of the failure of a cover to be properly locked to the pod base. 
     Another object of the invention is to provide a pod of the type mentioned above in which positive locking of the cover on the pod base is confirmed by an indicator light on the pod. 
     Another object of the invention is to provide a pod as described above wherein the state of the cover latching mechanism is positively displayed in view of an operator. 
     A still further object of the invention is to provide a pod as described above in which the indicator employs a latch sensing system that is simple and economical in design, and which may be retrofitted to existing pods. 
     These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which form an integral part of the specification, and are to be read in conjunction therewith, and in which like components are designated by identical numerals in the various views: 
     FIG. 1 is a front, elevational view of a semiconductor wafer pod according to the prior art; 
     FIG. 2 is a bottom plan view of a semiconductor carrying pod in accordance with the preferred embodiment of the present invention; 
     FIG. 3 is a fragmentary, front view of the pod of FIG. 2; 
     FIG. 4 is a schematic diagram of the sensing and indicator circuit that form part of the pod of FIG. 2; 
     FIG. 5 is a diagrammatic view showing the translation of mechanical movement between the drive member and latch members; 
     FIG. 6 is a diagrammatic view of the electrical switch forming part of the indicator employed in the pod of the present invention, the ultimate position of the driving pin being indicated in the phantom; 
     FIG. 7 is a front view of the pod of the present invention, parts being broken away in section for clarity; 
     FIG. 8 is a view similar to FIG. 2 but showing the details of the bottom of the pod, a cover plate having been removed therefrom; 
     FIG. 9 is a bottom view of the pod shown in FIG. 7; and, 
     FIG. 10 is an exploded, perspective view of the base forming part of the pod. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, in accordance with the prior art, a pod includes a generally rectangular base  15  upon which there is supported a cassette  12  containing a plurality of vertically spaced, semiconductor wafers (not shown) disposed in stacked relationship to each other. The pod also includes a cover  10  disposed over and protectively enclosing the cassette  12 . The cover  10  is provided with a peripheral flange  17  which engages an edge of the base  15 , so that the cover  10  is carried on the base  15 . A tag  36  is secured to one face of the cover  10  and is used to identify the pod and/or its contents. The tag  36  may comprise a simple bar code device or, an electronic device that can be automatically read by an electronic reader. 
     The cassette  12  is held on the base  15  by means of an H Bar  18 . The H Bar  18  forms a part of the top side of the base, and cooperates to receive a locking member on the base of the cassette  12 , in a bayonet like manner. 
     The base  15  includes a pair of outwardly slidably latch plates  16  having outer fingers  26  which are received in indentations or other recesses (not shown) on the inside wall of the cover  10  in order to lock the cover  10  on the base  15 . The latch plates  16  are driven by a circular rotatable plate  24  having a pair of drive pins  28  on the outer periphery thereof. The drive pins  28  are received within elongated slots (not shown) respectively in the latch plates  16 . The circular plate  24  is in turn driven by a pair of latch pins  42  that are driven to rotate by other apparatus present at the processing station (not shown) where the pod is delivered in order to load or unload wafers therefrom. Rotation of the plate  24  causes the drive pins  28  to move the latch plate  16  outwardly so that the fingers  26  move into retainers or indentations in the cover  10 , thereby latching the latter in place. However, in the event of slight misalignment or tilting of the cover  10  relative to the base  15 , or where there is a failure of any of the mechanical driving components, the cover  10  may fail to be properly latched down onto the base assembly  15  but such latching failure is not readily detectable by an operator. 
     Referring now to FIGS. 2,  3 , and  5 , the present invention contemplates an improved pod of the type generally discussed with reference to FIG. 1, but containing several novel features that assure that improperly latched down covers  10  will not go undetected. As best seen in FIG. 2, the rotatable plate  24  is generally circular and contains the central latch pin hole  22  in the center thereof. Latch pins  32  are secured on the inner periphery of hole  22 . The drive pins  28  are fixed to the periphery of the circular plate  24 , on diametrically opposite sides of the latter. Drive pins  28  extend up through and are received within elongated slots  30  defined in the latch plates  16 . The application of a rotary force via latch pins  32  is transmitted through plate  24  to drive pins  28 . Rotation of latch pins  28  drives the latter to one end of the slots  32 . As plate  24  continues to rotate, drive pin  28  force latch plates  16  outwardly, thus converting the rotation of drive force applied to latch pins  32 , to linear sliding motion of latch plates  16 . This transitional motion is diagrammatically shown in FIG.  5 . The rotational motion of plate  24  indicated as the link  50  and  52  is translated to linear motion of the link plate  16 . 
     Additional details of the pod are shown in FIGS. 7-9. As seen in FIG. 7, the cassette  12  is provided with a female locking member  54  which is received within a bayonet-like fixture  18  defined on the upper surface of the base plate  16 . The bottom of the base  15  is enclosed by a cover plate  20  having openings through which latch pins  32  and registration pins  60  extend. As shown in FIG. 8, mechanism for rotating the drive pins  32  include a worm gear  68  engaging a gear member  35 . 
     In accordance with the present invention, sensing means are provides for sensing when the cover  10  is positively latched down on the base  15 , and for providing a visual indication of whether the cover is in its latched, locked position, or its released, open position. The sensing means includes a pair of electrical switches  34  (FIGS. 2,  4 , and  6 ) which respectively utilize the drive pins  28  as a moveable switch element for establishing a circuit between two electrical contacts that may be in the form of, for example, a pair of flat conductors  34   a ,  34   b  which overlap at  34   c . The contacts  34  and  34   b  are carried on the latch plates  16 , adjacent to slots  30  so that drive pins  28  act as wipers which respectively shift, as plate  24  rotates, from a first position  29   a  (FIG. 6) in which the switch is open, to a second position  29   b  in which the electrically conductive pin  28  makes simultaneous contact with the contacts  34   a ,  34   b , thereby closing the circuit between these two contacts. 
     As shown in FIG. 4, contact  34   a  is coupled through a resistor  44  to a light emitting diode (LED)  40 , while contact  34   b  is likewise coupled through a resistor  46  as well as a capacitor  48  to the LED  40 . The LED  40  is coupled in series with a DC voltage source  42  which is in turn connected with the drive pin  28  which forms part of the switch  34 . The DC power source  42  may comprise a simple battery mounted on a convenient location on the base  15  of the pod. Preferably, the battery  42  along with the remainder of the circuit shown in FIG. 4 is mounted on a circuit board  38  secured to the base  15 . The LED  40  is likewise secured to an exterior surface of the base  15  so as to be readily visible to an operator. 
     FIG. 10 depicts certain details of one embodiment of the base assembly  15 . As previously noted, the base plate  16  has formed on the top thereof the H Bar mount  18  for holding the cassette  12 . Sandwiched between the base plate  16  and bottom plate  20  is the circular drive plate  24  which is journaled for rotation in a hub  64  that is secured within an annular recess  62  of the support  60 . 
     Both the rotating plate  24  and the latch plate  16  are formed of an electrically nonconductive material, thus insulating the switch  34 , including the drive pins  28 , from the surrounding components. 
     From the foregoing, it may be appreciated that the improved pod described above not only provides for the reliable accomplishment of the objects of the invention, but it does so in a particularly economical and efficient manner. It is recognized, of course, that those skilled in the art may make various modifications or additions chosen to illustrate the invention without departing from the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.
A pod for transporting a cassette containing semiconductor wafers includes a base and a cover removably carried on the base for protectively enclosing the cassette. A latching mechanism for latching the cover on the base is provided with an electrical sensor that senses when the cover is properly latched on the base and controls an electrical light on the base to provide a visual indication of latch status.