| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系所 === 102 === This dissertation contains two parts. In the first part, we control the formation of η-Cu6Sn5 by adopting <111> oriented and nanotwinned copper pads. In 3D IC packaging, there are tens of thousands of microbumps in one single chip. Therefore, how to control the properties of the microbumps becomes an important issue. We report here an experimental approach for controlling the microstructure of η-Cu6Sn5 intermetallic compound in microbumps by using <111> oriented and nanotwinned Cu pads as the under-bump-metallization (UBM). By electroplating arrays of large numbers of <111> oriented and nanotwinned Cu pads and by electroplating the Sn2.3Ag solder on the pads, we form η-Cu6Sn5 in the reflow at 260 oC for 1 min. The η-Cu6Sn5 showed a highly preferential growth along the <0001> direction. As reflow time is extended, the preferred texture of η-Cu6Sn5 changed to {21 ̅1 ̅3}. These results indicate that we can control the uniform microstructure of η-Cu6Sn5 IMCs by controlling the microstructure of the Cu under-bump-metallization.
In the second part, we attempt to inhibit the formation of tin whiskers by manufacturing a uniform layer of IMCs between solder and copper trances. Whiskers have been one of the most persistent reliability issues in packaging industry since it was discovered in 1940s. After decades of research, the diffusion of Cu atoms into tin layers is considered as one of the driving force for whisker formation. In this study, to inhibit the formation of whiskers, a novel method to hinder the diffusion of copper atoms in Cu-Sn couple was introduced. A uniform Cu6Sn5/Cu3Sn of intermetallics was formed in only two minutes of reflowing with limited volume of tin. The uniform intermetallic layers between copper and tin were served as a barrier layer. In as-electroplated samples without this uniform layer, whiskers were prone to grow on the surface of tin in one to two weeks at room temperature storage. In contrast, the samples with the uniform intermetallic layers between copper and tin were whisker-free even after 7 months of storage. The cross-sectional areas of both conditions were observed. It was found that a fast growth of Cu6Sn5 along grain boundaries of tin occurred in samples without intermetallic layers. The diffusivity of copper in Cu6Sn5, Cu3Sn, and tin has been investigated in previous studies. It is reported that the inter-diffusivities of copper in Cu6Sn5 and Cu3Sn are 7-8 orders smaller in magnitude than that in Sn. Therefore, this IMC barrier layer can effectively block the flux of copper atoms, and eventually inhibit the formation of Sn whiskers.
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