Effect of wafer back grinding on the mechanical behavior of multilayered low-k for 3D-stack packaging applications

To study the effect of back grinding on the mechanical properties of the active side of the die, low-k stacked wafers were grinded to four different thicknesses of 500 mum, 300 mum, 150 mum, and 75 mum by using a commercial grinding process. Nanoindentation and nanoscratch tests were performed using the Nanoindenter XP (MTS Corp. USA) on both the normal (no back grinding) and back grinded samples to analyze the failure loads, 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 in terms of the higher failure load and cohesive and/or adhesive strength of grinded low-k stack than the normal low-k stack. The TEM cross-section analysis showed that the interfaces in the low-k stack of normal sample are wavy, whereas the interfaces in the low-k stack of back grinded samples are even, especially at the black diamond region. Significant densification of BD films is 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 and enhance the mechanical strength of the low-k stack.