Abstract:
A guard fence is mounted on a transfer arm used to transport a boat carrying semiconductor wafers. The fence limits tipping movement of the boat and includes a plurality of sensors to sense tipping movement of both the top and bottom of the boat, in opposite tipping directions. A guard cover includes sensors that sense when the boat is tipped off-axis before the boat is moved into a process chamber by a boat elevator. Each of the sensors delivers signals to a controller which controls the operation of the transfer and elevator and interrupts operation thereof when the boat has tipped off-axis.

Description:
TECHNICAL FIELD 
     The present invention broadly relates to apparatus for moving and transporting boats carrying semiconductor devices, such as wafers, and deals more particularly with apparatus for sensing and controlling the tipping motion of a boat during movement thereof. 
     BACKGROUND OF THE INVENTION 
     A boat is often used in semiconductor manufacturing processes as a carrier for holding and transporting a plurality of semiconductor devices, such as wafers. The boats are transported using various types of specialized handling equipment, such that an entire batch of wafers carried on a boat is processed at the same time. One particular set of handling equipment is employed to move a boat between a standby or transfer position, to a loading position from which equipment, through a series of mechanized movements, transports the boat into a processing station, such as a furnace, for example, where a diffusion operation is performed. Following the simultaneous processing of all the wafers carried on a given boat in a single batch processing operation, the boat is withdrawn in a reverse set of movements by the handling equipment to a transfer station where the boat is transported by other transport mechanisms to another processing station. 
     The boat handling equipment referred to above typically comprises a boat transfer unit, a boat elevator, a boat clamp and a boat inserter controller. The boat transfer unit, which includes a transfer arm on which the boat sits, transfers the boat from the wafer transfer position, to the loading position vertically aligned above the boat elevator. The boat elevator moves upwardly to contact and engage the boat, whereupon the boat is clamped to the elevator which then moves the boat upwardly into a process chamber. The boat transfer arm typically includes a series of stops and alignment pins which help locate and stabilize the boat on the arm, however the boat is not securely clamped to the arm, and thus is subject to tipping as a result of numerous influences such as vibrations or jolting caused by the equipment or by events such as earth tremors. 
     Since the boat transfer sequence, like most operations during the fabrication process, is carried out completely automatically by computer controlled robotic mechanisms, it has not been possible to detect when a boat has tipped on a transfer arm to any degree. As a result, where tipping forces are severe, the entire boat may tip over during the transfer process, thus resulting in damage to the wafers, equipment down time and related reduction in throughput. Even where forces causing a boat to tip are not severe, the tipping motion may be sufficient to move the boat out of its normal on-axis alignment position on the transfer arm. This slight off-axis tilt of the boat can result in the boat not moving squarely into the process chamber, but rather colliding with the chamber walls or a scavenger cover disposed adjacent one end of the chamber. This collision, again, can result in process interruption requiring human intervention, as well as wafer damage. 
     It is apparent then that there is a clear need in the art for apparatus for both sensing when a boat has tipped beyond a threshold level requiring attention, as well as for preventing severe tipping or toppling over of a boat on a transfer arm. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, apparatus is provided for preventing tipping of a semiconductor boat carried on a moveable transport arm, which comprises fence structure mounted on the arm that extends upwardly from the arm and partially surrounds the boat such that the fence engages the tipping boat and limits tipping movement of the boat. The fence structure includes an arcuate fence mounted above the arm by vertical supports, and several displaceable guard sections which are resiliently mounted so as to absorb shock when engaged by a tipping boat. A plurality of sensors sense the displacement of each of the guard sections, and send signals to a boat transfer mechanism controller when the boat engages one of the guard sections. A second set of motion sensors are mounted on the arm to sense tipping movement of the base of the boat. Finally, a third series of sensors are mounted on a displaceable cover positioned above the boat elevator, and are operative to send tilt detect signals to the controller when the elevator attempts to move an off-axis, tilted boat upwardly into a process chamber. 
     According to another aspect of the invention, apparatus is provided for controlling the tipping movement of a boat carried on a transport arm comprising sensor means responsive to the tipping movement of the boat for producing an electrical signal indicative of the tipping movement, and control means coupled with the sensor means and responsive to the electrical signal for controlling the operation of the elevator. The sensor means comprises a first set of sensors carried on a fence which limits the movement of the boat, that produce tip detection signals indicating tipping movement of an upper portion of the boat. The sensor means includes a second set of sensors mounted on the transfer arm to sense tipping movement of the bottom of the boat in an opposite direction and are also operative to send tip detection signals to the controller. A third set of sensors mounted above the boat elevator sense engagement of the boat with a scavenger cover when the elevator attempts to move an off-axis, tilted boat into a process chamber. 
     In their preferred form, the sensors comprise simple limit switches which include displaceable mechanical elements that produce an electrical signal when actuated by the tipping movement of the boat. The controller typically comprises a programmed computer control, such as a PLC which is responsive to any of the sensors to interrupt both transfer and movement operations so as to avoid damage to the boat or wafers. 
     It is therefore a primary object of the present invention to provide apparatus for sensing and controlling the tipping movement of a semiconductor boat carried on a transfer arm. 
     Another object of the invention is to provide apparatus as described above which physically limits the tipping motion of the boat on the arm. 
     Another object of the invention is to provide apparatus as aforementioned which senses tipping motion either a lower or upper portion of the boat, in either of two directions. 
     A still further object of the invention is to provide apparatus as described above which includes a controller for automatically interrupting the boat transfer process when boat tipping is sensed, thereby to prevent damage to the boat or wafers. 
     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. 
    
    
     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 reference numerals designate identical components in the various views: 
     FIGS. 1-4 are perspective views of a boat and related boat transfer mechanism in successive stages of operation; 
     FIG. 5 is a perspective view of the boat transfer arm, the guard fence and sensors not shown for sake of clarity; 
     FIG. 6 is an enlarged, side view of the transfer arm having the novel sensor means of the present invention mounted thereon, a boat being shown carried on the arm. 
     FIG. 7 is a plan view of the transfer arm having the fence and sensor mounted thereon, the position of the boat being indicated in the phantom; 
     FIG. 8 is a perspective view of the scavenger cover having a sensing plate mounted on the bottom thereof; and, 
     FIG. 9 is a combined block and schematic diagram of the circuit for sensing and controlling boat tipping and boat transfer movement which forms a part of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIGS. 1-4, the present invention relates to apparatus for moving a boat generally indicated with the numeral  10  containing semiconductor devices, such as wafers  18 , in a lateral direction from a wafer transfer position, to a loading position axially aligned and immediately above a boat elevator  22 , which is motorized to move vertically upwardly. The elevator  22  includes a base  20  upon which the boat  10  is clamped after the latter is moved to a position vertically aligned above the elevator  22 . 
     The boat  10  includes a circular base and top,  12  and  14  respectively which are interconnected by four vertical struts or supports  16 . The interior faces of supports  16  contain grooves to receive the edges of the wafers  18  so as to hold a plurality of the wafers in vertically spaced apart relationship. The boat  10  is moved from its wafer transfer position to a loading position immediately above the elevator  22  by means of a transfer arm  24  which is coupled with a motorized transfer mechanism  26  that moves the arm  24  laterally between the two operative positions, respectively associated with the wafer transfer and loading positions referred to above. In operation, the wafer transfer arm picks up the boat  10  at the wafer transfer position and transfers it laterally to the loading position immediately above the elevator  22 . Then, the elevator  22  moves upwardly until the base  20  engages the bottom of the bottom wall  12  of the boat  10 , whereupon the boat  10  is clamped to the base  20  and the transfer arm  24  is moved away, back to its wafer transfer position. Continued vertical movement of the elevator  22  causes the boat  10  to move upwardly through an opening  56  and an upper wall  54  of an enclosure until the boat  14  moves into a processing chamber  55 . After a processing step is conducted on the wafers  18  held by the boat  10 , the elevator  22  is then again actuated so as to move the boat  10  downwardly so as to withdraw the boat  10  from the chamber  55 . The elevator  22  has moved to its fully lowered position, the arm  24  moves laterally to pick up the boat  10  and move it back to its wafer transfer position. 
     Referring now also to FIGS. 5-9, the transfer arm  24  includes a generally “C” shaped section  28  which transitions on one end thereof to a connector arm  26  that is adapted to be secured, as by screws to a connector  38  forming part of the motorized transfer arm mechanism. The transfer arm  24  includes three spaced apart boat supports  30 , preferably of a quartz material which form the sole points of contact and support for the boat  10 . As best seen in FIG. 6, the boat supports  30  contact the lower face of the base  12  of the boat  10 . In order to assure that the boat  10  is properly registered on arm  24 , a pair of alignment pins  32 - 34  are provided, also preferably made of quartz. Finally, a generally rectangular stop  36  is provided on the top surface of the arm  24 , which includes a straight inner face that engages a corresponding straight edge of the bottom wall  12  of boat  10 . The stop  36  in combination with the alignment pins  32 - 34  engage the outer edge of base  12  at three spaced apart locations so as to properly locate the boat  10  on the arm  24 . 
     As previously discussed, since the boat  10  merely sits on top of the arm  24 , supported only at three points (supports  30 ), the boat  10  is subject to tipping during movement from the wafer transfer position to the loading position. In accordance with the present invention apparatus is provided for sensing and controlling the tipping motion of the boat  10 , which broadly comprises fence structure that includes a generally arcuate, “C” shaped fence  40  supported in spaced relationship above the arm  24  by means of three spaced apart, vertical struts or supports  42 . The upper ends of supports  42  are secured to the fence  40  as by welding, and the lower ends of the supports  42  are secured to the outer edge of the arm  24 , as by screws  44 . On the interior face of the fence  40  there is provided three guard fence sections  42 ,  44 , and  46  which are resiliently mounted as by spring elements  48  to the interior face of the fence  40 , so as to be inwardly displaceable against the biasing influence of the springs  48  in the event that the boat  10  tips into engagement with any one of the guard sections  42 - 46 . 
     Also mounted on the exterior face of the fence  40  are three spaced apart sensors which may be in the form of limit switches L 1 , L 2 , and L 3 . Limit switches L 1 -L 3  are mechanically connected to an associated one of the guard sections  46 ,  44 ,  42  so as to be actuated when the corresponding guard section is inwardly displaced upon engagement therewith of the boat  10 . Each of the limit switches L 1 -L 3 , when actuated, closes a normally open switch which sends an electrical signal to a boat transfer control unit  57 . Limit switches L 1 -L 3  therefore sense tipping movement of an upper portion of the boat  10  in any of three general directions, all of which are generally inwardly toward the fence  40 . In order to sense outward tipping of the boat  10 , away from the fence  40 , there are provided a pair of limit switches L 4 , L 5 , which are mounted on and circumferentially spaced around the perimeter of the “C” shaped section  28  of the arm  24 . As best seen in FIG. 6, limit switches L 4 , L 5  each include a mechanical actuator disposed immediately above an edge of the bottom wall  12  of boat  10  so as to be contacted upon outward tipping of the boat  10 . Closure of either of the limit switches L 4 , L 5  closes a switch, as best seen in FIG. 9 which sends an electrical signal to the boat transfer unit controller  57 . 
     From the above, it may be appreciated that the invention not only provides a fence  40  for limiting tipping movement of the boat, but also provides sensing means for delivering electrical signals to the controller  57  providing an indication that tipping movement is occurring. The controller  57  is responsive to such signals to stop movement of the transfer arm  24  and/or the elevator  22  so as to minimize the possibility of the boat  10  toppling over. As a further means of sensing undesirable tipping movement of a boat, a plate  54  having a central aperture  56  therein is resiliently mounted by means of springs, for example, on the bottom wall of the scavenger cover  52 . Four additional limit switches L 6 , L 7 , L 8 , and L 9  are provided to sense upper deflection of any of the four edges of the plate  54 . Such deflection of the plate  54  may occur when the boat has tipped slightly to an off-axis position causing the upper extremities of the boat to engage the bottom surface of the plate  54  rather than passing through the aperture  56 . Upon actuation of any one of the limit switches L 6 -L 9 , a signal is delivered as seen in FIG. 9, to the controller  57 , which reacts by immediately stopping the elevator  22  so that no damage is done either to the boat  10  or the elevator mechanism  22 . 
     From the foregoing, it may be appreciated that the apparatus for sensing and controlling the tipping of a semiconductor boat as described above not only provides for the reliable accomplishment of the objects of the invention, but does so in a particularly effective and economical manner. It is recognized, of course that those skilled in the art may make various modifications or additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of 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.