Electron backscatter diffraction study for advanced semiconductor interconnect materials analysis

Abstract

3-dimensional (3D) semiconductor packaging and interconnection technology, which involves vertically stack IC chips that are electrically interconnected by inter-layer vias and solder joints, is being used to achieve better performance, lower power consumption, and higher integration capability. As interconnects continue to shrink and become denser, it is increasingly important to characterize their microstructure such as grain size and crystallographic orientation, to ensure that these interconnects, are robust and able to survive thermo-mechanical stresses during assembly, reliability testing as well as in field usage. Furthermore, interlayer dielectric (ILD) cracking and delamination still remains a concern. The use of brittle extreme-low k (ELK) dielectrics together with lead-free tin-silver-copper solder and stiff copper (Cu) pillars can increase the risk of this occurrence. A possible solution to mitigate this risk is to use a lower modulus solder. However, this may favor the formation of brittle intermetallic compounds (IMCs) in the solder joints and compromising the overall reliability performance of devices. Therefore, a study of the IMCs in the solder joints would be necessary.

Electron backscatter diffraction (EBSD) is a technique which can be used to understand the crystallographic structures of the IMCs, which may influence its reliability. For EBSD analysis to be productive, useful data that allows the crystallographic information to be extracted from must first be obtained from the sample. To achieve this, the analyzed sample has to be optimally prepared. As conventional sample preparation methods for electron microscopy inspection may not be sufficient for the stringent conditions that EBSD samples require, a modified sample preparation workflow is explored and validated as well.

A study of the substrate side IMCs was carried out on SAC305 and Sn0.7Cu solders. A comparison of the IMCs microstructure was done under temperature cycling as well as high temperature storage with the use of energy dispersive x-ray spectroscopy (EDX) and EBSD techniques. Results showed that no significant difference was observed between the IMCs of both substrate solders, validating Sn0.7Cu solder as a viable alternative to conventional SAC305 solder in mitigating ILD cracks and delamination risks.