Filter Structure for Sub-Micron Filtration Fabricated in Silicon

The design and fabrication of a new particle filter in the sub-micron range are described. The filtration principle is based on two laterally displaced silicon hole membranes, through and between which the fluid has to pass. The thickness of the silicondioxide spacers separating the hole membranes defines the size of the largest particles which can pass through the structure. The filter structure was fabricated using a special two-step self-aligning technique where one single hole pattern, heavy boron diffusion, anisotropic silicon etching and silicondioxide undercut-etching constituted the most important process steps. The fact that electrically the filter structure is a capacitor makes other applications possible. Structures with membrane separation distances of 50 and 200 nm have been made.