The influence of sub-surface damage microstructure on ultra-thin die flexural strength

In the semiconductor industry, where miniaturization is a key driver, mechanical properties of ultra-thin dies are increasingly important research topics. Sub-surface damage (SSD) is a common issue in wafer thinning processes, but there is a lack of research on the relationship between SSD microstructure and ultra-thin die strength. In this study, the influence of SSD microstructure on flexural strength was investigated through three-point bending tests of ultra-thin dies prepared by distinct wafer-thinning methods, coupled with SSD microstructure characterization. Flexural strength was highest for dies dry polished with N pad, intermediate for dies dry polished with M pad, and lowest for dies with fine grinding. We researched SSD microstructure by high-resolution transmitted electron microscope (HRTEM), revealing that it comprises amorphous regions, micro-cracks, and high-density distortion areas. The SSD of the fine grinding samples was thick and intermittent, with observable micro-cracks. Comparatively, the SSD structure from M pad polishing was uniform but thicker, whereas SSD from N pad polishing was thinner but exhibited greater variability. SSD thickness not only influences the average value but also dictates the distribution of flexural strength. This research enhances the understanding of SSD microstructure's impact on ultra-thin die flexural strength, providing valuable insights for optimizing wafer thinning processes to enhance die reliability.