PATENT NUMBER: 6,507,092 ASSIGNEE: Fujitsu Ltd. INVENTORS: Norio Fukasawa, Takashi Hozumi and Toshimi Kawahara TITLE: Semiconductor Device Having Increased Reliability and Method of Producing the Same ... One approach to building a wafer-level package consists of applying a layer of resin to the wafer's surface for environmental protection. A problem that occurs in that process, however, is the occurrence of a TCE mismatch between the resin and the underlying silicon, which can cause the resin layer to separate from the silicon surface. Contaminants A factor aggravating the TCE problem is that the wafer surface may contain contaminants that make it difficult for the seal resin to form a strong bond.
The contaminants on the surface are typically residue from the wafer manufacturing processes. They can be impurities from the diffusion process, thin film layers, photolithography residue and even polyimide resin, which is employed to seal the wafer's surface. These contaminants are either too difficult to remove or are part of the normal surface and thus cannot be removed.
One approach to the problem has been to form a groove in the scribe area, as shown in the inset figure. The groove-forming operation is normally done after the wafer has been bumped and before the resin layer is applied. The groove may have a rectangular or triangular shape based on the saw blade's shape. After the groove is formed, the resin coating is applied and cured. The wafer is then diced using a thinner saw blade so that resin remains in the grooved area. The adhesion of the resin to the grooved area is much higher, reducing the tendency of the resin film to delaminate. Although this is an improvement over previous methods, the dicing process inevitably generates stresses in the grooved regions. Subsequently, during temperature cycling, any cracks that occur during dicing tend to propagate. The TCE difference between the resin and the silicon may increase the stress on a crack. If adhesion is lost between the resin and the groove, the delamination effect continues because of the low adhesion to the contaminated wafer surface. The Invention The low adhesion problem has been solved by using a suitable laser system to remove the contamination from the wafer's surface between the bond pads of adjacent chips. The laser may be CO2-, excimer- or YAG-based, and will not induce cracking in the wafer surface. The laser beam also does not clean the surface where active circuitry resides. The laser cleaning of the surface, however, does remove some material from the wafer surface. The result is a slightly downset area that is roughened and which forms a high strength bond to the resin encapsulation. The key is to have a wide, laser-cleaned area and a narrow saw blade to dice the wafer. The final step in the process is to dice the wafer into individual packages. The amount of resin in contact with the cleaned area provides sufficient strength to provide a reliable seal.
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