Studies of Silane Adhesion Promoters on Silica Filler Particles for use in Microelectronic Packaging
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Studies of Silane Adhesion Promoters on Silica Filler Particles for use in Microelectronic Packaging
Maura Jenkins, Gretchen DeVries, Reinhold H. Dauskardt, and John C. Bravman Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, U.S.A. ABSTRACT To avoid delamination which often occurs in microelectronic packaging as a result of widely varying thermal expansion coefficients, epoxy underfill is often loaded with particulate silica filler. This filler typically complicates the fracture behavior of the epoxy and surrounding layers. However, delamination of interfaces within the composite may contribute to toughening if the fracture path is adhesive between the epoxy and surrounding layers or otherwise removed from filler-epoxy delamination regions. In this work, the interface between silica filler and epoxy is altered with the use of silane coupling agents to elucidate the fracture and toughening mechanisms of filled epoxies. Two model epoxies are investigated, and coupling agents were chosen to encourage chemical reaction with each system. For the aliphatic epoxy system, coupling agents were identified which both augment and degrade the interface between filler and epoxy. In composite structures, it was observed that those coupling agents which degrade the filler-epoxy interface act as toughening agents. SEM indicates that most deformation is not at the filler-epoxy interface, but rather through the epoxy, very close to the filler particle surfaces. In contrast, no coupling agents were identified which act as toughening agents in the bisphenol-f system. Further, no variation in fracture toughness of composite structures was observed with the use of coupling agents, indicating that filler-epoxy delamination does not occur in this system. With few exceptions, SEM supports this interpretation. INTRODUCTION Adhesion of bimaterial interfaces in microelectronic packaging is of critical importance, as delamination degrades the performance and reliability of the entire device. Of particular concern is the wide variation in thermal expansion coefficients (CTEs) of the commonly used materials. To address this, epoxy underfill is often loaded with silica particulate filler to adjust its CTE to match that of the surrounding layers. As in any composite, this new structure introduces deformation processes which complicate the expected mechanical behavior of the engineered product. These processes need to be studied in order to improve both microelectronic packaging and integrated circuit reliability. The adhesion of microelectronic packaging interfaces can be significantly improved through the use of adhesion-promoting organic films. For example, previous work at Stanford University [1] has shown that the use of γ-aminopropylsilane and vinylsilane coupling agents significantly improves the adhesion of benzocyclobutene (BCB) polymer to silicon oxide. Chemistry plays an important role in the efficacy of these coupling agents. Widely varying fracture toughness values have been obtained through var
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