Abstract:
In accordance with the foregoing objects and advantages, the present invention provides a fabrication device that may be used during the grinding operation of the fabrication process. The fabrication device comprises a socket plate that includes a plurality of cavities formed therein that correspond in position and number to the solder (or other conductive material) bumps formed on the front surface of a product wafer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field Of Invention 
   The present invention relates generally to semiconductor wafers, and more particularly to the devices and methods for fabricating wafers having solder bumps formed on one surface thereof. 
   2. Background 
   The desire for smaller, thinner, lighter electronic components has driven the semiconductor packaging industry to develop tools and processes for wafer backside thinning by grinding and polishing before the wafers are diced into chips. As packaging assemblers develop more sophisticated multi-chip packages containing multiple die which are often stacked in a 3D assembly, the need for thinner die becomes more acute. 
   Wirebond stacked die packages may contain die thinned to less than 200 um, with even more aggressive thinning anticipated in the future. Because the surface of a wafer being processed for wire bonded interconnects is planar, the mechanical process of backside grinding and surface finishing is capable of removing greater than 90% of the original wafer thickness without breakage or lattice damage. These techniques have enabled the manufacture of extremely thin stacked wirebond package assemblies for advanced electronics. 
   With increasing semiconductor complexity comes the desire for greater signal bandwidth, which requires that the semiconductor chips have higher interconnect density from the die to the module substrate, or in the case of stacked assemblies, from die to die. Higher performance interconnects are achieved by switching from wirebond to solder bump connections. However, since the solder bumping process must be done with unthinned wafers, the 30–100 um solder bump feature creates significant challenges to obtain wafer backside thinning comparable to what is achieved today for wirebond wafers. 
   Thinning of wafers with solder bumps is done today by using a compliant tape on the wafer front side to conform around, cushion, and protect the solder bumps during the grinding process. This is reasonably effective for final wafer thicknesses greater than 300 um. For thinner dimensions, a tape with higher compliancy allows for replication of the underlying bump features especially in dissimilar pattern density regions of the bumps. Conversely, a tape with lower compliancy may also reflect pattern density differences as well as posing difficulty in removal. These issues have prevented the packaging industry from embracing thin single and multi-chip stacked die packaging for solder bumped wafers. 
   3. Objects and Advantages 
   It is therefore a principal object and advantage of the present invention to provide a fabrication device and method for permitting the effective thinning of bumped wafers through a grinding operation. 
   It is another object and advantage of the present invention to provide a fabrication device that permits backside grinding of bumped wafers to thinner dimensions than is presently feasible. 
   It is a further object and advantage of the present invention to provide a fabrication device that is easily adapted to existing fabrication processes. 
   Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter. 
   SUMMARY OF THE INVENTION 
   In accordance with the foregoing objects and advantages, the present invention provides a fabrication device that may be used during the grinding operation of the fabrication process. The fabrication device comprises a socket plate that includes a plurality of cavities formed therein that correspond in position and number to the solder (or other conductive material) bumps formed on the front surface of a product wafer. Prior to the back-grinding process that thins the product wafer before it is diced into chips, the front surface of the product wafer is placed into engagement with the socket plate with each of the solder bumps being positioned in a respective socket. The dimensions of the sockets are slightly larger than the dimensions of the bumps in order to provide a protective housing. When the bumps are positioned within the sockets, the front surface of the product wafer contacts the opposing surface of the socket plate. The backside of the product wafer may then be ground without exerting any excessive force on the bumps. In addition, the socket plate maintains the product wafer in a stable position during the grinding operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
       FIGS. 1   a  and  1   b  are front elevation, enlarged, cross-sectional views of the preferred and alternate embodiments of the present invention, respectively; 
       FIG. 2  is a perspective view of a typical mask used in connection with the present invention; and 
       FIGS. 3A–3D  are sequential, front elevation, cross-sectional views of the socket plate of the present invention in its various stages of fabrication. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawing figures in which like reference numeral refer to like parts throughout, there is seen in  FIG. 1  a fabrication system, designated generally by reference numeral  10 , employed during the grinding operation of a wafer fabrication process in order to thin a product wafer  12  prior to its being diced into chips. Product wafer  12  is of the type that includes a plurality of conductive bumps  14  formed in an array on its front surface  16  in a predetermined pattern, and an opposing back surface  18  that is adapted to be engaged by a grinding pad during the grinding operation of the fabrication process in order to thin the wafer to a predetermined thickness (generally of about 300 um). 
   Bumps  14  may be formed on front surface  16  by using a metal mask  20  that includes apertures  22  formed therethrough that are of about the same dimension and arranged in the same predetermined pattern as is desired for the bumps (the bumps are of a height, h, generally on the order of 50 um–100 um, and a diameter, d, generally on the order of 60–120 um). 
   Fabrication system  10  further comprises a socket plate  24  that includes a front surface  26  through which (See  FIG. 1   a  illustrating holes formed entirely through plate  24 ) or in which (see  FIG. 1   b  illustrating blind holes formed in the front surface  26  of plate  24 ) a plurality of cavities (or “sockets” and which may be either through holes (see  FIG. 1   a ) or blind holes (see  FIG. 1   b ))  28  are formed. Cavities  28  are of height, H, and diameter, D, that are slightly larger than the height, h, and diameter, d, of product wafer  12 , and are equal in number and arranged in the same predetermined pattern as bumps  14 , thereby permitting bumps  14  to be inserted into corresponding cavities  28  for purposes described hereinafter (bumps  14  are generally in the range of 60–120 um in diameter and 50–100 um high, and cavities  28  should be about 10–30 um greater in those dimensions than bumps  14 ). 
   Socket plate  24  may be fabricated from a silicon wafer, but may also be fabricated from a metal plate, such as one made from Molybdenum or stainless steel, or a plastic plate that may be etched. Preferably, plate  24  is also relatively thin (on the magnitude of about 0.1–1 mm in thickness), and is round with about the same diameter as bumped product wafer  12 . 
   The following paragraph describes a method of fabricating a socket plate out of a silicon wafer utilizing the same Molybdenum mask used to form the solder bumps (C 4  balls) on the product silicon wafer. Cavities  28  are formed by first coating a CVD (chemical vapor deposition) oxide layer  30  with a positive resist material  32 , such as TOK3227 manufactured by Tokyo Ohka Kogyo Co., Ltd. (See  FIG. 3A ). Next, mask  20  is flipped from its orientation used to create bumps on surface  16  and is placed over CVD oxide layer  30 . The resist  32  is exposed through mask  20  and then removed by the developer, thereby leaving openings  34  that correspond to the pattern of bumps  14  (See  FIG. 3B ). CVD oxide layer  30  is then etched using resist material  32  as a mask. The resist material  32  may then be removed, leaving an oxide hard mask (see  FIG. 3C ). A deep silicon etch may then be performed (if pate  24  is made of silicon) on the socket plate  24  until the cavities  28  are formed to a depth H deep enough to receive bumps  14  therein (see  FIG. 3D ) (as stated earlier, the cavities  28  may be formed entirely through plate  24  or partially through plate  24  with the important feature being that the depth H and diameter D are sufficient to accommodate bumps  14 ). If a material other than silicon is used for plate  24 , such as metal or plastic, cavities  28  may be formed using well known, traditional etching techniques for metal plates, or by mechanical or laser drilling operations. 
   Prior to grinding the back surface  18  of product wafer  12 , socket plate  24  is placed on a vacuum chuck  36  and is put into registry with product wafer  12  with each bump  14  being positioned within a corresponding cavity  28 , with front surface  16  of product wafer  12  contacting surface  26  of socket plate  24 . Back surface  18  may then be ground in the typical manner. The support provided to product wafer  12  by socket plate  24 , permits the grinding force to be applied to back surface  18  evenly and prevents bumps  14  from incurring excessive forces from the grinding operation. It should be noted, however, that with respect to the preferred embodiment illustrated in  FIG. 1   a , the vacuum created by vacuum chuck  36  is effective at maintaining bumps  14  in position within cavities  28  due to the free flow of air through the cavities  28 , whereas with the embodiment of  FIG. 1   b  bumps  14  are maintained in stable position within cavities  28  due primarily to the force created by the grinding device.