Abstract:
A wafer cassette. At least one support bar includes a composite curved holding portion. Multiple separation plates are connected to the composite curved holding portion and are parallel to and separated from each other.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a wafer cassette, and more particularly to a wafer cassette carrying wafers. 
         [0003]    2. Description of the Related Art 
         [0004]    During a semiconductor manufacturing process, transportation of wafers is accomplished by a wafer cassette. Namely, multiple wafers are supported in the wafer cassette and transported thereby to predetermined process machines or destinations for subsequent processing. 
         [0005]    Referring to  FIG. 1A  and  FIG. 1B , a conventional wafer cassette  1  is composed of PP or PEEK and comprises two opposite support bars  11  and a plurality of separation plates  12 . The separation plates  12  are respectively connected to the support bars  11  and parallel to and separated from one another. When the wafer cassette  1  carries a plurality of wafers (not shown), the wafers are respectively separated by the separation plates  12  and supported by the support bars  11 . 
         [0006]    As shown in  FIG. 1B , as the support bars  11  provide a strip-like construction, linear contact exists between the wafers and the support bars  11 . Specifically, when a wafer W is vertically placed into the wafer cassette  1  from the top thereof and pulled downward by gravity, as shown in  FIG. 2 , linear impact (shown by area A) is generated between the wafer W and the support bars  11  with the strip-like construction. Here, as the hardness of the wafer W exceeds that of the support bars  11  composed of PP or PEEK and a large contact stress (such as 16.943 kgw/mm 2  obtained from an experimental result) is generated due to a small contact area between the wafer W and the support bars  11  with the strip-like construction, the support bars  11  are easily damaged by impact and wear, causing cassette particles. The cassette particles from the support bars  11  then fall onto the surfaces of the wafers or process machines during transportation of the wafer cassette  1  and wafers, thus causing damage to the wafers during subsequent processing. For example, if the cassette particles fall onto the top surface of a wafer, partial circuits cannot be laid on dies of the wafer by obstruction of the cassette particles when the dies are subjected to an exposure process, thereby causing die deficiency at the related pattern exposure areas. In another aspect, if cassette particles fall onto the bottom surface of a wafer, exposure areas protrude from the wafer due to the cassette particles when the bottom surface of the wafer is attached to a flat wafer chuck, causing local defocus of the exposure areas, and further causing die deficiency at the related pattern areas. 
         [0007]    Moreover, as the large contact stress generated between the wafer W and the support bars  11  with the strip-like construction can be further transmitted to the center or other portions of the wafer W, metal conducting wires on dies of the wafer W are easily broken by the presence of the large contact stress, causing deficiency of the dies. 
         [0008]    Hence, there is a need for a wafer cassette utilizing a special composite curved holding portion to support wafers. A contact stress between the composite curved holding portion and the wafers is effectively reduced, significantly reducing the amount of cassette particles produced, and further enhancing overall process yield of the wafers. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
         [0010]    An exemplary embodiment of the invention provides a wafer cassette comprising at least one support bar and a plurality of separation plates. The support bar comprises a composite curved holding portion. The separation plates are connected to the composite curved holding portion and are parallel to and separated from each other. 
         [0011]    The composite curved holding portion comprises a first curved surface, a second curved surface, and a third curved surface. The second curved surface connects the first curved surface to the third curved surface. 
         [0012]    The direction of curvature of the second curved surface is opposite to that of the first curved surface. The direction of curvature of the third curved surface is the same as that of the first curved surface. 
         [0013]    The first, second, and third curved surfaces comprise a plurality of round surfaces, respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0015]      FIG. 1A  is a schematic perspective view of a conventional wafer cassette; 
           [0016]      FIG. 1B  is a schematic top view of  FIG. 1A ; 
           [0017]      FIG. 2  is a schematic partial cross section and side view of the conventional wafer cassette carrying a wafer; 
           [0018]      FIG. 3A  is a schematic perspective view of a wafer cassette of an embodiment of the invention; 
           [0019]      FIG. 3B  is a schematic top view of  FIG. 3A ; 
           [0020]      FIG. 4  is a schematic partial cross section and side view of a wafer cassette of an embodiment of the invention; and 
           [0021]      FIG. 5  is a schematic partial cross section and side view of a wafer cassette, of an embodiment of the invention, carrying a wafer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0023]    Referring to  FIG. 3A  and  FIG. 3B , a wafer cassette  100  may be composed of PP or PEEK and comprises two opposite support bars  110  and a plurality of separation plates  120 . 
         [0024]    As shown in  FIG. 3B , each support bar  110  comprises a composite curved holding portion  111 . Specifically, as shown in  FIG. 4 , each composite curved holding portion  111  comprises a first curved surface  111   a,  a second curved surface  111   b,  and a third curved surface  111   c.  The second curved surface  111   b  connects the first curved surface  111   a  to the third curved surface  111   c.  More specifically, the direction of curvature of the second curved surface  111   b  is opposite to that of the first curved surface  111   a,  and the direction of curvature of the third curved surface  111   c  is the same as that of the first curved surface  111   a.  In this embodiment, the profiles of the first curved surface  111   a,  second curved surface  111   b,  and third curved surface  111   c  are designed by means of the finite element method, and the first curved surface  111   a,  second curved surface  111   b,  and third curved surface  111   c  thus comprise a plurality of round surfaces, respectively. 
         [0025]    As shown in  FIG. 3B , the separation plates  120  are respectively connected to the composite curved holding portions  111  of the support bars  110  and are parallel to and separated from each other. When the wafer cassette  100  carries a plurality of wafers (not shown), the wafers are respectively separated by the separation plates  120  and supported by the composite curved holding portions  111  of the support bars  110 . 
         [0026]    Accordingly, as the composite curved holding portions  111  of the support bars  110  are designed by a special manner, surface contact exists between the wafers and the composite curved holding portions  111 . Specifically, when a wafer W is vertically placed into the wafer cassette  100  from the top thereof, as shown in  FIG. 5 , large-area contact (shown by area B) is generated between the wafer W and the composite curved holding portions  111 . More specifically, by the special design of the first curved surface  111   a,  second curved surface  111   b,  and third curved surface  111   c,  a fixed large contact area (such as an optimal contact area of 13 mm×0.75 mm obtained from experimental results) can be maintained between the wafer W and the composite curved holding portions  111 , such that the wafer W is always subjected to minimal stress when contacting the composite curved holding portions  111 . Accordingly, as a reduced contact stress (such as 13.617 kgw/mm 2  obtained from an experimental result) is generated due to an increased contact area between the wafer W and the composite curved holding portions  111 , cassette particles are not easily generated by wear of the composite curved holding portions  111  even though the hardness of the wafer W exceeds that of the composite curved holding portions  111  composed of PP or PEEK. Thus, during transportation of the wafer cassette  100  and wafers, the amount of the cassette particles from the composite curved holding portions  111  is significantly reduced, effectively preventing deficiency of the wafers during subsequent processing. 
         [0027]    Moreover, because reduced contact stress is generated between the wafer W and the composite curved holding portions  111  due to the optimal contact area, the reduced contact stress is not easily transmitted to the center or other portions of the wafer W, thereby preventing breakage of metal conducting wires on dies of the wafer W by the presence of the stress, and further enhancing yield of the dies of the wafer W. 
         [0028]    In conclusion, as the disclosed wafer cassette effectively provides a reduced contact stress between the composite curved holding portions (or support bars) and the wafers, the amount of the cassette particles is significantly reduced, thus enhancing overall process yield of the wafers. 
         [0029]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.