A micromachining technique for a thin silicon membrane using merged epitaxial lateral overgrowth of silicon and SiO₂ for an etch-stop

A micromachining technology using merged epitaxial lateral overgrowth (MELO) of silicon, combined with SiO₂ as an etch-stop, was developed. When epitaxial lateral overgrowth (ELO) silicon merges on top of SiO₂ islands, it forms a local silicon-on-insulator (SOI) film which can be transformed into a thin silicon diaphragm needed for micromechanical sensors. The SiO₂ islands then act as a near-perfect etch-stop during back etching due to its negligible etch rate in KOH- or ethylenediamine-based solution. This technique permits the formation of both transducers and transistor structures in the same film since it does not require a highly doped film for the etch-stop. The diaphragm thickness is controlled by the epitaxial silicon growth rate (≅0.1 μm/min) rather than by traditional etching techniques. A single-crystal silicon diaphragm 9 μm thick and 250 μm × 1000 μm was realized. Diodes fabricated in the MELO silicon showed characteristics similar to those in bulk silicon, including reverse bias leakage currents.