Void formation in Cu-Sn Micro-Connects

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Sähkötekniikan korkeakoulu | Master's thesis
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Master's Programme in Micro and Nanotechnology
Degree programme
EST - Master’s Programme in Micro and Nanotechnology (TS2005)
6 + 84
The implementation of micro-connects is the next evolutionary step in fabricating high density integrated circuits (IC) and microelectromechanical systems (MEMS). The shift to micro-connects as an interconnect technology not only substantially enhances the packaging density of ICs but also improves performance. As attractive as this technology sounds, there are significant reliability challenges which need to be overcome. The interfacial voiding of Cu-Sn micro-connects is one such challenge and is the focus of this thesis work. Voiding has been identified as a challenge due to the negative impact on the mechanical and electrical performance of interconnections. As micro-connects have very small feature sizes, the impact of interfacial voiding on reliability is more significant. Previous studies have identified electroplated Cu as a common source of voiding. Additionally electroplating parameters affect the propensity of an interface to exhibit voiding. Voids observed in a Cu-Sn micro-connect are located within the intermetallic compound (IMC) of Cu3Sn or at the interface of Cu and Cu3Sn. A review of the published literature leads to a common scientific consensus that voids exhibiting this behaviour are Kirkendall voids. Kirkendall voids form due to the imbalance of atomic flux during solid state diffusion. As will be discussed, despite the fact that diffusion appears to be a driving force for void formation, this alone cannot be the only factor in void formation. This work will systematically study the average void density and size as a function of thermal annealing. Samples will be prepared using a range of electroplating parameters, including electroplating chemistries and current densities. Additionally, the impact of voids on the diffusion mechanisms and the IMC growth rates will be presented. The motivation of this work is to present quantitative data which can be used for both an understanding of electroplating parameters on the voiding propensity, and for the reliability impacts of voids on micro-connects.
Paulasto-Kröckel, Mervi
Thesis advisor
Vuorinen, Vesa
micro-connects, voiding, reliability, electroplating, impurities, diffusion