Modeling Stress in Silicon With TSVs and Its Effect on Mobility

With the most popular electronic products being the slimmest ones with the highest functionality, the ability to thin, stack, and interconnect chips is becoming more important. One method to accomplish this is by using the through silicon via (TSV). This is a means of electrical connection in 3-D stacked devices that saves space and shortens the electrical interconnect length, improving electrical performance. Unfortunately, the large mismatch between the coefficients of thermal expansion (CTE) of copper (17.5 × 10^-6/°C) and silicon (2.5 × 10^-6/°C) has made the TSV a reliability concern. A mismatch in CTE translates to a mismatch in thermal strain when the wafer is subjected to large temperature loadings during fabrication. This local thermal mismatch also induces stresses on the silicon surface around the vias which can affect the mobility of the silicon. In this paper, the thermal stresses and strains induced on silicon due to the proximity of copper vias have been investigated for various geometries (via diameter, via pitch, silicon thickness, stacking layers) using finite element modeling. These results should be useful for: 1) designing substrates with TSVs such that mobility in the active devices are not affected by the presence of TSVs, and 2) understanding the limitations of stacking chips with respect to stress in silicon as well as joint reliability.