BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a wafer cassette, and more particularly to a wafer cassette carrying wafers.
2. Description of the Related Art
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.
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.
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/mm2 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.
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.
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
A detailed description is given in the following embodiments with reference to the accompanying drawings.
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.
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.
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.
The first, second, and third curved surfaces comprise a plurality of round surfaces, respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1A is a schematic perspective view of a conventional wafer cassette;
FIG. 1B is a schematic top view of FIG. 1A;
FIG. 2 is a schematic partial cross section and side view of the conventional wafer cassette carrying a wafer;
FIG. 3A is a schematic perspective view of a wafer cassette of an embodiment of the invention;
FIG. 3B is a schematic top view of FIG. 3A;
FIG. 4 is a schematic partial cross section and side view of a wafer cassette of an embodiment of the invention; and
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
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.
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.
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 111a, a second curved surface 111b, and a third curved surface 111c. The second curved surface 111b connects the first curved surface 111a to the third curved surface 111c. More specifically, the direction of curvature of the second curved surface 111b is opposite to that of the first curved surface 111a, and the direction of curvature of the third curved surface 111c is the same as that of the first curved surface 111a. In this embodiment, the profiles of the first curved surface 111a, second curved surface 111b, and third curved surface 111c are designed by means of the finite element method, and the first curved surface 111a, second curved surface 111b, and third curved surface 111c thus comprise a plurality of round surfaces, respectively.
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.
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 111a, second curved surface 111b, and third curved surface 111c, 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/mm2 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.
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.
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.
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.
Patent Application
20090184028
