Microporous silicon for CMOS compatible MST

The use of selectively formed microporous silicon as a sacrificial layer in silicon micromachining has been studied. Porous silicon formation, by isotropic etching in hydrofluoric acid (HF) can be selective, by creating n-type areas on a p-type substrate, or also, using polycrystalline silicon under adequate conditions. Porous silicon can be removed easily by CMOS compatible chemicals. Using this technology, free standing membranes, cantilevers, bridges and microchannels, can be fabricated. Porous silicon etching is much easier and faster than crystalline silicon isotropic etching, and both fabrication and etching of the porous film can be performed at, or nearly at room temperature. Thick layers of porous silicon can be easily produced. The formation conditions of the microporous silicon layer and the etch conditions for microstructures fabrication are described.