MICROMACHINING APPLICATIONS OF POROUS SILICON

Besides its exciting electro-optical properties, porous silicon shows some interesting features for micromechanical applications. Owing to the doping sensitivity of the porous silicon formation, a wide range of pore and crystallite sizes can be formed easily. Because of their high aspect ratio, macropores from n-type silicon can be used for the etching of trenches or vias into the wafer. Mesoporous silicon created from heavily doped silicon has a smooth surface suitable for further thin-film processes, such as silicon nitride and carbide deposition, to be performed on it. In the smallest size range, nanostructures are formed with structure sizes beyond 10 nm without electron-beam lithography. Another important point is the high surface-to-volume ratio. After impregnation of the porous layer with a catalyst, if can be applied in gas and humidity sensors. For the most common micromachining applications, free-standing membranes, cantilevers and bridges are needed. In contrast to the surface micromachining method, where a thin film of silicon dioxide (< 10 mu m) is dissolved as a sacrificial layer, by means of porous silicon technology, thick sacrificial layers can be produced (up to 100 mu m). This results - after their dissolution - in a high distance of membranes and bridges to the bulk silicon, which is an important aspect for the sensitivity of thermal transducers. The capabilities of porous silicon application in micromachining will be demonstrated by some examples, e.g. thin-film bolometers and polysilicon hot-wire anemometers.