Abstract:
An apparatus for stacking semiconductor wafers comprises a housing configured to releasably maintain a plurality of semiconductor wafers in fixed positions relative to the housing. The apparatus also includes a transfer guide proximate to the housing, the transfer guide configured to facilitate the transfer of the plurality of semiconductor wafers into the housing. Also included is a member configured to detach the semiconductor wafers from the housing so as to collect the semiconductor wafers into a stack.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
   This invention relates generally to semiconductor wafer fabrication. More specifically, this invention relates to a method and apparatus for stacking semiconductor wafers. 
   BACKGROUND OF THE INVENTION 
   Cassette carriers are often used to transport finished semiconductor wafers prior to dicing. However, these carriers suffer from certain drawbacks. Current cassette carriers maintain excessive space between wafers, taking up too much volume in transport and resulting in unnecessary expense. This is especially the case in situations involving scrap wafers, when wafer-to-wafer contact is of little concern. Wafers are also subject to handling damage when they are removed from cassette carriers, resulting in reduced process yields and other inefficiencies. 
   Therefore, it would be highly desirable to stack wafers more compactly, so that they occupy less volume in transport. It would also be desirable to stack wafers in a manner that minimizes handling damage. 
   SUMMARY OF THE INVENTION 
   In one embodiment, an apparatus for stacking semiconductor wafers comprises a housing configured to releasably maintain a plurality of semiconductor wafers in fixed positions relative to the housing. The apparatus also includes a transfer guide proximate to the housing, the transfer guide configured to facilitate the transfer of the plurality of semiconductor wafers into the housing. Also included is a member configured to detach the semiconductor wafers from the housing so as to collect the semiconductor wafers into a stack. 
   In another embodiment, an apparatus for stacking semiconductor wafers comprises a housing having supports for releasably engaging a plurality of semiconductor wafers, and a basket configured to receive the plurality of semiconductor wafers. The basket is further configured to disengage the wafers from the supports, and to manipulate the wafers into a stack. 
   The invention also includes methods of stacking semiconductor wafers. In one such embodiment, a method of stacking semiconductor wafers comprises positioning a plurality of semiconductor wafers proximate to a housing, the housing having supports configured to releasably hold the plurality of semiconductor wafers. The plurality of semiconductor wafers is transferred into the housing so that the wafers are releasably held within the housing, and the plurality of semiconductor wafers is released from the supports so as to collect the plurality of semiconductor wafers into a stack. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates a cutaway side view of a wafer stacking apparatus constructed in accordance with an embodiment of the invention. 
       FIG. 2  illustrates a top view of a wafer stacking apparatus constructed in accordance with an embodiment of the invention. 
       FIG. 3  illustrates a cutaway side view of a wafer stacking apparatus as operated in accordance with an embodiment of the invention. 
   

   Like reference numerals refer to corresponding parts throughout the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a cutaway side view of a wafer stacking apparatus constructed in accordance with an embodiment of the invention. The wafer stacking apparatus  10  includes a housing  20  sized to hold a number of semiconductor wafers  30  inside, a number of supports  25 , a receptacle  40  that holds a wafer cassette  50 , and a transfer guide  60 . The apparatus  10  can also include a transfer arm  70 , as well as a basket  80  attached to a lift  90  that is powered by a lift motor  100 . 
   In operation, a wafer cassette  50 , which can be simply a standard wafer cassette commonly used during semiconductor fabrication operations, is placed within the receptacle  40 . The wafer cassette  50  holds a number of semiconductor wafers  30  and is typically designed with openings to allow the entry of a transfer arm  70 . The transfer arm  70  then moves in the direction shown by the arrow  110 , so as to push the wafers  30  through the transfer guide  60  and into the housing  20 , where they rest on the supports  25  in fixed positions inside, and relative to, the housing  20 . Once the wafers  30  are positioned on the supports  25 , they can then be stacked. In this embodiment, the lift motor  100  is employed to move the lift  90  and basket  80  in the direction of the arrow  120 . This moves the basket  80  through the housing  20  and in contact with the wafers  30 . Continued movement of the basket  80  in the direction of the arrow then disengages the wafers  30  from the supports  25 , where they then drop down into the basket  80 . In this manner, the basket  80  can disengage all the wafers  30  within the housing  20 , and collect them into a stack. The basket  80  and stack of wafers  30  can then be detached and removed from an opening in the housing  20 , or the wafers  30  can simply be removed and the basket  80  returned to its original position in preparation for another batch of wafers. 
     FIG. 2  illustrates a top view of the wafer stacking apparatus  10 , which further highlights the operation of the transfer guide  60  and supports  25 . In one embodiment, the transfer guide  60  simply provides a set of slots  65  of sufficient length L to keep the wafers  30  oriented generally (in this embodiment) horizontally as they are slid from the wafer cassette  50  into the housing  20 . In this manner, the slots  65  act to direct each wafer  30  onto the correct supports  25 . As the slots  65  come in direct contact with the wafers  30 , they are preferably made of a material that will not damage the wafers  30 . For example, any surfaces of the slots  65  that might contact a wafer  30  can be fabricated from a non-abrasive and cleanroom-compatible material such as nylon, or a cleanroom-compatible plastic. The slots  65  and supports  25  are also spaced sufficiently far apart from one another to keep the wafers  30  from contacting each other. 
   The supports  25  hold the wafers  30  up and in a stationary position once they are placed within the housing  20 . As can be seen from  FIG. 2 , the supports  25  are located so as to keep wafers  30  in a fixed position, so that they do not fall out of the housing  20 . In the embodiment of  FIG. 2 , three supports  25  are employed for each wafer  30 , but the invention employs any number of supports  25  sufficient to maintain wafers  30  in fixed positions. 
   The supports  25  are designed to releasably contact the wafers  30 , without adhering to them. The supports  25  are also designed to release the wafers  30  upon pressure applied to the wafers  30 . In this manner, supports  25  hold the wafers  30  for stacking without risking handling damage. 
     FIG. 3  illustrates a cutaway side view of the apparatus  10 , for purposes of illustrating further details of the supports  25  and how they facilitate the stacking of wafers  30 . As mentioned previously, the wafers  30  are pushed from the wafer cassette  50  through the slots  65  in the wafer guide  60 , and onto the supports  25  within the housing  20 . Each wafer  30  then rests releasably on the supports  25 . Pushing the basket  80  through the housing  20  brings it into contact with successive wafers  30 , disengaging them from the supports  25  and allowing them to fall into the basket  80 . The basket  80  can include guide posts  130  for manipulating the disengaged wafers  30  into a stack configuration. The invention thus acts to stack wafers into a more compact configuration than the wafer cassette  50 , while subjecting them to minimal handling. 
   The supports  25  can take on a number of configurations while remaining within the scope of the invention. For example, the supports  25  can be made of a known flexible material such as a malleable plastic. In this configuration, the supports  25  would flexibly uphold the wafers  30 , yet would yield to the basket  80  and the stacked wafers  30  when they are pushed past the supports  25 . As the supports  25  in this configuration are flexible, they will typically brush past the outer edges of each wafer  30  in the stack as the basket is pushed through the housing  20 . Choosing a material of sufficient pliability, such as any one of several known flexible cleanroom-compatible plastics, will allow the supports  25  to deform enough to avoid damage to the outer edges of the wafers  30 . Alternatively, the supports  25  can be pivotably attached to the housing  20  so that contact with the basket  80  or stack of wafers  30  simply pivots the supports  25  away from the wafers  30 . For example, the supports  25  can be designed with a hinged end close to the surface of the housing  20 , so that they pivot away from the wafers  30  in response to pressure. Both the flexible and pivotable configurations of the supports  25  are contemplated by the invention. 
   Once the wafers  30  are detached from the supports  25  and collected into a stack in the basket  80 , they can be manually removed from an opening in the housing  20  (such as from the top), and the basket can then be lowered back through the housing  20  in preparation for stacking more wafers. It can be observed from  FIG. 3  that the stacking of wafers  30  can result in wafer-to-wafer contact within the stack. Such contact may be acceptable when stacking scrap wafers for disposal or recycling, and is contemplated within some embodiments of the invention. However, such wafer-to-wafer contact may not always be desirable, for example when the invention is used to stack production wafers. In such a case, the invention includes the use of carbon paper or other material such as spacers that can be placed on the supports  25  prior to inserting the wafers  30  into the housing  20 . Wafers can then be simply slid through the transfer guide  60  and onto the carbon paper/spacer, where stacking would then result in a stack comprising alternating layers of carbon paper (or other material) and wafers  30 . 
   The foregoing description, for purposes of explanation, uses specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. For instance, it has been emphasized above that the invention includes multiple configurations of support structures. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.