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Intermetallic Compound Formation in Scaled Solder Joints used for 3D Silicon-to-silicon Stacking

Boek - Dissertatie

The third dimension, or vertical dimension, of integrated circuits, attracted more interest in the recent years since it allows achieving device density multiplication by stacking IC layers in the third dimension. To increase the functional density and obtain higher computing performance, the interconnects used in 3D integration, such as TSVs and microbumps, must be reduced in size in a reliable way. The decrease of the interconnect size and increase of the interconnect density can bring new challenges for process integration and reliability. This dissertation is therefore mainly focusing on the process development and reliability evaluation for miniaturized interconnected solder joints. The dissertation starts with a detailed introduction of 3D integration as well as an overview of different state of the art bonding/assembly approaches. Following that, different bonding/assembly approaches are compared, and their advantages and drawbacks are discussed. The motivation and challenges for the development of miniaturized solder joints are also discussed.The microstructure of the microbumps during the product-service stage is not in the thermodynamic equilibrium state after the bonding process. At evaluated temperatures and with current flow, a new intermetallic compound (IMC) phase will replace the initial UBM and solder. This phase change affects the overall performance, including the thermal, electrical and mechanical properties, of a solder joint and can lead to failures.With the demand for miniaturization in the modern electronics industry, the understanding of the interactions between the materials involved in the solder joint is extremely critical for reliability. In order to predict the phase transformations and the growth rate of the intermetallic materials, the electrical and kinetics parameters for different metallurgy systems, including Co/Sn, Ni/Sn, Cu/Ni/Sn and Ni/Cu/Sn, were extracted using a novel methodology, i.e. in-situ resistance measurement. This gives insight into the interfacial reaction. Based on these measurements, a recommendation of the best metallurgical system for miniaturized interconnects is given.Besides the intermetallic growth kinetics for alternative metallurgical systems, there were also other reliability concerns related to continuing downsizing the solder joint. One of the most important ones is the impact of process variation, affecting the Sn grain size inside the solder joint. The results of a phase-field simulation study of the effect of Sn grain size on IMC morphology and solid-state interfacial reaction are discussed in this thesis. The simulation results give insight into the interfacial reaction for different metallurgical systems.Another large challenge for miniaturized solder-based stacking is the high risk of bridging issue between the neighboring solders due to the downsizing of interconnects pitch size. Therefore, a novel low-temperature bonding strategy to avoid the solder bridging issue for fine pitch solder joints is proposed and developed. In addition, results from first reliability tests on the micro-bump structures with the recommended metallurgical system are presented and discussed.Finally, conclusions and perspectives for further development of the fine pitch solder joint are presented.