Study on the Effect of Wafer Back Grinding Process on Nanomechanical Behavior of Multilayered Low-k Stack

This paper presents the effect of back grinding on the mechanical properties of the active side of the multilayered low-k stacked die. Low-k stacked wafers were thinned to four different thicknesses of 500, 300, 150, and 75 μm by using a commercial grinding process. Nanoindentation and nanoscratch tests were performed on both the normal (no back grinding) and back grinded samples to analyze the failure strength, modulus, hardness and adhesive/cohesive strength of the low-k stack. It is found that the back grinding process enhances the mechanical integrity of low-k stack as the back grinded low-k stack exhibited improved fracture load and cohesive and/or adhesive strength as compared to the normal low-k stack. The transmission electron microscopy cross-section analysis showed that the interfaces in the low-k stack of normal sample are wavy, whereas the interfaces in the grinded low-k stack samples are even, especially at the Black Diamond (BD), low-k region. Significant densification of BD films was observed in the case of back grinded sample. Based on these results, it is believed that the thermo-mechanical stresses applied and/or generated during wafer back grinding process affect the microstructure of low-k stack and thus enhance the mechanical strength of the low-k stack.