Silicon membrane nanofilters from sacrificial oxide removal

Silicon micropore filter designs using a sacrificial oxide removal technique are described. These filters utilize surface and bulk micromachining for precise control of pore sizes in the tens of nanometers range. The semipermeable membrane of the sacrificial layer filters (SLF's) is typically composed of sandwiched p ⁺ polysilicon/oxide/p ⁺ silicon layers where the sacrificial oxide between the two silicon layers determines the pore size of the filter. The purpose of this paper is to address special design and fabrication considerations for control of pore size uniformity within 10%, adapting surface conditions for filtration fluxes of deionized water to fall in the range of 1 ml/cm ²h, and maximization of the structural response of SLF's. Fluorescent beads are used to analyze the pore size uniformity of fabricated filters, with achievements of four-log reduction in fluorescent bead concentration.