UCLA

As the demand for wearable devices and mobile technology is increasing rapidly, a smaller but more powerful integrated circuit (IC) is needed. However, as the chip technology has reached its limit to extend Moore’s law, the 3D packaging technology is currently the most promising way to improve transistor density while fulfilling the downsizing requirement of a silicon chip. Compared to currently applied 2D IC system, one of the most distinctive structures to realize the vertical stacking in 3D IC is the use of microbumps to join the through-Si vias (TSV) vertically. The diameter of microbumps (<10 μm) is much smaller than the traditional control-collapse-chip-connection (C-4) flip chip solder joints (~100 μm) and ball-grid-array joints (BGA, ~760 μm). The dramatic change in solder joint size brings new challenges in two aspects: (i) The intermetallic compound (IMC) formed in solder joint will eventually turn into only one grain structure from a polycrystalline structure. Structural anisotropy begins to strongly affect microstructure evolution kinetics and properties. (ii) The surface-to-volume ratio is greatly increased. In this thesis, a systematic study of Cu-Sn surface interdiffusion is conducted on both 30 �m wide Cu-Sn free surface which is fabricated by FIB and Sn/Cu pillars with diameter of 2 �m. A 2-step surface IMC formation mechanism is proposed to explain the protrusion of surface IMC formation on Cu-Sn surface. In the first step, the surface IMC forms much faster on the surface of Cu3Sn layer than on any other layers. In the second step, the surface IMC will form on Cu surface. The IMC forms in the first step will have a potential threat to 3D IC reliability when the diameter of the microbump is below 2 �m. A simple kinetic model of surface diffusion-controlled surface IMC growth at Cu3Sn layer is proposed which explains the tendency of the lateral IMC growth, and it can be used to estimate the failure time for 3D IC which is caused by contact shorting due to the surface IMC growth.