A novel micromachining process for the fabrication of monocrystalline Si-membranes using porous silicon

We report a new surface micromachining technology to fabricate monocrystalline silicon membranes covering a vacuum cavity for applications like piezoresistive pressure sensors. The main process steps are: (i) local anodic etching of layered porous silicon with different porosities, (ii) thermal rearrangement of the porous silicon, and (iii) epitaxial growth of the silicon membrane layer. In contrast to conventional bulk micromachining the new technology has the benefit of a considerable freedom in the design of mono-crystalline silicon membranes. The membrane geometry is only determined by the porous region. Further, the new fabrication method is fully CMOS compatible. In fact, except for anodic etching, all process steps are part of a standard mixed signal IC production line. Various aspects of the used key process steps are discussed, particularly with regard to the oxygen and fluorine desorption during the porous silicon annealing. A piezoresistive pressure sensor with integrated ASIC based on the new fabrication method is demonstrated.