Abstract: Methods for fabricating a semiconductor device are provided. In one embodiment, the method includes forming a Sub-Isolation Buried Layer (SIBL) stack over a semiconductor substrate. The SIBL stack includes a polish stop layer and a sacrificial implant block layer. The SIBL stack is patterned to create an opening therein, and the semiconductor substrate is etched through the opening to produce a trench in the semiconductor substrate. Ions are implanted into the semiconductor substrate at a predetermined energy level at which ion penetration through the patterned SIBL stack is substantially prevented to create a SIBL region beneath the trench. After ion implantation, a trench fill material is deposited over the SIBL stack and into the trench. The semiconductor device is polished to remove a portion of the trench fill material along with the sacrificial implant block layer and expose the polish stop layer.