高密度电流下TSV结构Sn基微焊点组织不均匀性演化规律及其对可靠性的影响

Three dimensional integrated circuit manufacturing based on the through silicon via (TSV) technology is a more cost-effective solution for high density electronic packaging. However, the decreasing dimension in 3D integration systems has not only led to very limited number of grains, the high volume fraction of intermetallic compound (IMC) and defects like voids in TSV solder interconnect, but resulted in the very high electric current density carried by the interconnects, which can seriously deteriorate the reliability of solder interconnects. This project will firstly study the evolution of the interfacial IMC layer and the directional migration of diffusion atoms in TSV solder interconnects induced by the inhomogeneity of under bump metallization (UBM) and the solder matrix inhomogeneity as well as their coupling effect under high density current stressing. The main purpose is to reveal the micromechanism how the inhomogeneous microstructure of TSV solder interconnects influences the diffusion and migration of atoms. By characterizing the microstructure evolution of IMC layers with different initial thicknesses at both anode and cathode, the intrinsic correlation among the crystallographic characteristics of the major phases, thickness of IMC layers and the atomic diffusion flux of TSV solder interconnect under high density current stressing will be analyzed and established. Then the micromechanism of the effect of voids’ features on the atomic diffusion will be studied by investigating the microstructure evolutions around the void undergoing current stress. Finally, the influence of the inhomogeneous microstructure evolution on the reliability of TSV solder interconnects under high density current stressing will be comprehensively discussed and understood. It is envisaged that the results obtained from this study will offer a data reference and theoretical basis for construction design, process optimization and reliability assessment of TSV solder interconnects in advanced electronic packaging.

基于硅通孔（TSV）结构的三维封装是解决高密度封装问题的有效途径，但其极小尺度不仅导致TSV微焊点组织中晶粒稀少及出现高比例金属间化合物（IMC）和气孔缺陷等，还引起微焊点承载电流密度急剧增加而严重削弱焊点可靠性。本项目通过研究电流作用下TSV结构微焊点金属基底（UBM）组织不均匀性和焊点基体组织不均匀性以及两者之间的耦合作用对焊点组织演化和原子传输的影响规律，揭示微焊点中组织不均匀性影响原子扩散迁移的微观机理；然后研究两侧界面具有不同厚度IMC层TSV微焊点在电流作用下的演变行为，建立焊点组织中各物相晶体学特征、界面IMC尺度与原子扩散通量之间内在的量化相关性，并探明高密度电流下微焊点中气孔存在对周围组织及原子扩散迁移影响的微观机理；最后全面探究并掌握高密度电流下微焊点不均匀组织演变对焊点可靠性影响的规律。研究结果将为TSV结构微焊点结构设计和工艺优化及可靠性评估提供重要参考和理论基础。

本项目以实验和模拟手段揭示了具有TSV结构Sn基微焊点在高密度电流下不均匀组织的演化规律及其对可靠性的影响，主要研究内容包括：焊点基底组织不均匀性和焊点基体组织不均匀性以及两者耦合作用的影响，微焊点界面IMC层厚度差异的影响和高体积分数界面IMC层的演化，研究焊点不均匀组织的演化规律及其对原子扩散迁移规律和焊点可靠性变化的微观机理，影响微焊点润湿与铺展性能、缺陷及其形成原因、组织与力学性能的内在关系。主要研究成果包括：Cu晶粒（010）晶面垂直或近似垂直于电流方向时其局部溶解现象严重，邻近阴极的界面容易出现锯齿状溶解尖角，而（111）晶面垂直于或近似垂直于电流方向时Cu的局部溶解较为缓慢，阴极界面保持平整形貌；分析结果表明，（010）晶面较大的表面能导致的较高的原子释放率和较低的跃迁能垒是引起阴极基底锯齿状溶解的内在机制。受不同物相之间电阻率差别的影响，即使微焊点截面上输入电流密度一致，则经过不同的物相后电流分布会发生变化，在电阻率较小的物相中电流密度较高。通过研究过冷、共晶和正常回流温度条件下保温不同时间对IMC层厚度和Cu6Sn5晶粒尺寸的影响以及与微凸焊点剪切强度关系，发现随着保温温度提高和保温时间延长，IMC层厚度增加，但较薄IMC层导致了微凸焊点剪切断裂为平面滑动型，较厚IMC层则容易引起混合型断裂。进一步地，借助实验结果分析了影响类TSV结构微焊点成形能力的因素、锡溅与气孔之间和界面IMC层厚度与焊点抗冲击性能之间的关系；结果表明，焊盘表面污染物极大削弱熔融焊锡的铺展能力，导致润湿不良；焊盘内部微观空洞内气体受热膨胀后容易导致锡溅或气孔缺陷；由于尺寸较小的微焊点中脆性相IMC的比例较高，因此微焊点的抗冲击性能较差。上述成果为新型微焊点材料选择、结构与工艺优化和可靠性改善提供了理论依据和参考价值。