BDTI (backside deep trench isolation) structures parallel to a depth direction of a Si wafer may generate a stress concentration under the warpage caused by a mechanical loading on the wafer during handling or moving. Our work aims to provide a better understanding for minimizing a stress concentration effectively at the BDTI. To address shape factors of the BDTIs with a void, we change ? (open width), dBDTI (distance between Si-surface and BDTI bottom), ? (angle between Si-surface and sidewall of BDTI), and t1st HfOx (thickness of 1st HfOx film on silicon surface). Among the geometrical factors changed in this work, our simulation predicts that the opening length and the thickness of 1st HfOx films are key factors to control a maximum stress concentration at a 1st HfOx film below an oxide bottom. Comparing only a final geometry of various BDTIs, it is consequently very effective to decrease the interface curvature between an HfOx and oxide films in order to get low stress structures.