3D IC 封裝中接點空間受限誘發之界面反應關鍵效應

Abstract

Imminent ending of Moore’s law is the most critical issue threatening the continuing development of semiconductor industry. The strategy of consensus in order to go beyond Moore’s law is through the Three-Dimensional Integrated Circuit (3D IC) architecture. Among many 3D IC integration schemes under development today, solder micro-bumping is a very promising one. Due to the extremely small size of solder joints in 3D IC packages, interfacial reaction features are quite different to that in the conventional solder joints. The space confinement behaviour must be considered in the soldering reactions. In this thesis, six critical issues in soldering reactions arising from space confinement are proposed and discussed. The first issue arises from the concern that intermetallics (IMCs) may occupy a large portion of the solder joint volume. It is demonstrated that this concern is not only for Cu substrate but also for Ni under bump metallurgy which reacts very slowly with solders. The second issue relates to impingement and merging of IMC grains. When IMCs occupy a large portion of a joint, the IMC grains growing from the opposite sides of a joint will eventually impinge on each other. The grains originally with different orientations can merge into a single grain after the impingement. Grain coarsening is considered to be a key reason responsible for the merging behaviour. The third issue is about the solder volume effects on the IMC growth rate. It is demonstrated that solder volume has very little effects on the IMC species and their growth rates. The fourth issue is relating to effects of inert alloy constituents, i.e., Ag and Bi, in the space confined soldering reaction. The inert phase will be rejected from the interfacial reaction, and eventually locates near the centre of the joint as the solder is fully consumed. Experimental results also reveal that the inert phase can form as a continuous layer throughout the joint as the solder originally contains a higher inert constituent content. This morphology is considered having a great impact on the joint strength. The fifth issue arises from the fact that as size of joint becomes smaller, the surface area-to-volume ratio increases. This makes the impact of thin film layers on UBM and surface finish become ever higher. One well known example is Au. The so-called Au embrittlement issue may become relevant again in the 3D IC packaging. The upper limit of Au content for preventing Au embrittlement is estimated to be 1.3 wt.% in Sn. The last issue is the volume shrinkage during soldering reactions. It is the first time that the reaction induced volume shrinkage is experimentally measured in a Ni-Sn soldering reaction. Internal stress or structure defects may form because of this shrinkage. Theoretical analysis and experimental evidence are presented in this thesis to illustrate these issues, and implications based on the findings will be also discussed.