三维互连微凸点焊点早期界面反应与凝固行为及IMC生长的界面耦合效应

In recent years, the through-Si-via (TSV) and microbump interconnections, as emerging technologies for 3-dimensional (3D) integrated circuit manufacturing, are developing rapidly to satisfy the trend of miniaturization of the interconnects and high density packaging in modern advanced electronic devices and products. However, there is an increasing major reliability concern for the microbump interconnects with dramatic scaling down in dimensions, that is, the appearance of the increasing fraction of brittle intermetallic compounds (IMC) in the joints and the strong anisotropy of the joint solidification microstructure with limited grains have posted a serious challenge on reliability of the interconnects. The present project will explore new techniques and methods for investigating the novel early interfacial reaction and undercooling solidification behavior of microbump interconnects by means of differential scanning calorimetry (DSC) incorporated into the reflow soldering process. The focuses will be placed on clarifying the mechanism of the formation of IMC phases with strong anisotropy under heterogeneous nucleation condition during the early stage interfacial reaction integrating of solid-state to liquid-state, determining the boundary condition with which only limited grains will be formed in the microstructure of joints based on the study of the solidification behavior of the micro-joints at large undercoolings, establishing the intrinsic correlation between the interfacial restriction of micro-interconnects and characteristics of the undercooling and solidification microstructures, discussing the mechanism of interfacial-coupling effect on the growth and evolution of interfacial IMC in the micro-interconnect under external physical field loading and finally obtaining the coupling growth kinetics of the IMC during the whole life period from the soldering process to the real service situation. It is expected that the outcomes obtained from this project study will enrich the theoretical of interfacial physical metallurgy in low temperature soldering of metals under micro-scale and micro-environment conditions, and offer a theoretical guide for design, manufacturing and reliability assessment of microbumps and joints in advanced electronic packaging.

三维叠层封装硅通孔结构及微凸点互连技术可解决集成电路和电子器件尺寸日益微小及高密度封装问题，但互连尺寸呈数量级减小导致焊点中高比例脆性金属间化合物(IMC)及有限晶粒凝固组织呈显著各向异性进而严重影响焊点可靠性。本项目采用新颖研究方法，将焊点合金熔化凝固特性表征与回流焊工艺巧妙结合来研究微凸点焊点在微环境下特殊的早期界面反应与过冷凝固行为，通过研究焊点形成时固-固到固-液一体的早期界面反应揭示微尺度多界面结构IMC异质形核与各向异性长大机制；然后从焊点的大过冷凝固行为确立多界面约束下微熔体凝固过程有限晶粒形成的边界条件，并探索微焊点界面约束与过冷和凝固组织特征的内在关联；最后拓展到外加载荷下焊点IMC生长演化的界面耦合效应探讨，获得焊点IMC从工艺到服役全过程的耦合生长动力学规律。研究结果将丰富微尺度微环境下金属低温钎焊界面冶金理论，并为先进封装结构和工艺及可靠性设计提供技术理论支持。

本项目主要使用实验手段对无铅微互连结构焊点（百微米量级）和微凸点（十微米量级）形成过程中钎料熔体的凝固及焊点界面行为进行系统研究，主要研究内容包括：液相过冷回流制备Sn3.0Ag0.5Cu/Cu焊点及焊点凝固与界面冶金行为，界面局部熔化制备Sn3.0Ag0.5Cu/Cu焊点过程的组织演化规律及界面化合物生长的原位表征，界面结构约束下Sn0.3Ag0.7Cu/Cu微凸点界面行为的尺寸效应， Cu/Sn3.0Ag0.5Cu/Sn58Bi/Cu混装焊点低温形成过程与焊点力学可靠性。取得主要研究结果和创新之处包括：创新性地利用小尺寸无铅钎料具有大凝固过冷度并能稳定保持液态特点，设计并制备低温成型SAC305/Cu焊点，焊点内部和界面组织中IMC的凝固析出与过度生长行为得到抑制，获得了塑韧性更佳的微互连焊点。提出使用钎料熔化临界温度回流工艺或低熔点SnBi界面材料制备局部熔化互连结构焊点，并使局部熔化微互连焊点界面IMC的生长行为得到显著抑制，论证了SnBi合金应变速率敏感性和Sn3.0Ag0.5Cu钎料的软化效应是引起混装结构焊点断裂位置逐渐从SnBi/Cu界面转移到SAC305/Cu界面并由此诱发了焊点断裂模式从韧脆性断裂转变为韧性断裂的根本原因。创新性地使用原位观察手段对无铅微互连焊点界面IMC生长与演化过程进行动态观察，以实验手段直接观察并证实了焊点界面中大晶粒迅速生长而邻近小晶粒消失这一过程完全符合Ostwald 熟化理论所描述的演化特征。成功制备尺寸在10-50μm范围的Sn0.3Ag0.7Cu/Cu微凸点焊点，明确了微凸点界面Cu6Sn5晶粒生长主要受表面扩散控制，并进一步论证了微凸点界面IMC的主要生长机制是晶粒结合而非界面化合反应。本项目所发展的研究方法论及获得的研究结果把基于平面和三维芯片互连的无铅微焊点工艺性与力学可靠性方面的基本认识向前推进了一大步，为芯片互连结构的材料选择、设计优化、制备工艺以及使役性能评价等提供了重要理论指导，为今后三维芯片无铅微互连结构及封装系统的性能预测和可靠性评价提供了具有重要参考价值的方法和依据。