Porous silicon (PS) is an excellent material to be used as a sacrificial layer (SL) for the fabrication of membranes, bridges, cantilevers and other complicated micro-sensor structures. Major advantages of this material are its smooth surface and large area of pores that provide a high rate of PS SL removal. In this work, electrochemical and chemical etching processes of porous silicon formation from polycrystalline silicon deposited on insulating substrates have been studied. It was shown that a sufficiently thick (about 2 mu m), uniform PS with a smooth surface and high etching rate in buffered HF solution can be made by both electrochemical and chemical etching from low resistive p-type poly-Si only. Finally, the obtained PS layers have been used as sacrificial layers in a fabrication process of microbolometer array.