Fracture strength characterization and failure analysis of silicon dies

This paper characterizes fracture strength of a silicon die as a first step to predict and prevent die cracking during package assembly, reliability tests, and operation life. Die strength is measured via the three-point bend test conducted using a micro-force tester. Strength reduction due to surface defects, such as tiny notches or micro-cracks that are introduced through wafer backside grinding is evaluated. It is observed that die strength strongly depends on the grinding patterns, i.e. minimum die strength in a wafer is found if the grinding mark is in parallel with the loading axis. Furthermore, fracture strength of dies with different wafer surface conditions like polishing and no treatment (grinding) are also examined. Polished wafers possess the highest silicon strength owing to its minimum surface flaws. On the other hand, untreated wafers contain the most severe surface defects; hence exhibit the lowest die strength. Geometrical factors (square vs. rectangular) and die thickness (4 vs. 6 mils) are probed as well, however these factors do not contribute to die strength degradation. Surface morphology and roughness studies of silicon dies via scanning electron microscope and atomic force microscope also confirmed that die strength degradation is mainly controlled by surface defect (roughness) levels. Observed fracture modes also correlate well with measured die strength.