Porous-silicon technology is successfully used for frontside bulk silicon micromachining in a process which is fully C-MOS compatible. Porous silicon is used as a sacrificial layer and it is removed by C-MOS-compatible chemicals, leaving a very smooth bottom surface and sidewalls. Deep trenches, bridges with suspended membranes and cantilevers are formed, which open new possibilities in monolithic integration of sensors with electronics. Cavities as deep as ∼ 120 μm and suspended polysilicon membranes with a flat surface as large as 230 μmX 550 μm have been fabricated by this process. Also other micromechanical structures such as very flat polysilicon cantilevers of dimensions 150 μmX2 μmX2 μm are easily obtained after optimization of the process in order to minimize the strain within the polysilicon films.