A bulk silicon micromachining technique using porous silicon as a sacrificial layer is developed. The proposed process is fully C-MOS compatible and it was successfully used to fabricate deep cavities into silicon with very smooth bottom surfaces and sidewalls. Suspended flat polysilicon membranes were also produced of a surface as large as 230 × 550 μm2, as well as polysilicon cantilevers. This process opens important possibilities in silicon integrated sensor fabrication.