Porous Silicon in a Semiconductor Manufacturing Environment

In the Bosch-proprietary “advanced porous silicon membrane process”, porous silicon (PSi) is used for the first time in high-volume industrial production of microelectromechanical devices. Nanoporous silicon acts as an auxiliary layer during manufacturing of monolithically integrated pressure sensors in a mixed-signal IC process supplemented by microelectromechanical-systems-specific steps in the front end of line. In this paper, the technical design and performance of a fully automated production tool capable of high-volume fabrication of PSi under the specific constraints of a semiconductor manufacturing environment are discussed. The process requires stringent control on the PSi layer thickness, uniformity, porosity, and morphology. The impact of chamber and electrode geometry, the electrolyte flow, and the mode of current coupling into the wafer back side on the uniformity of the PSi layer is addressed. The need for a well-defined PSi morphology demands high reproducibility and stability of the electrolyte composition, particularly with respect to the hydrofluoric acid concentration.