Sharp silicon tips with different aspect ratios in wet etching/DRIE and surfactant-modified TMAH etching

A simple, high yield method of surfactant-modified wet anisotropic etching for the fabrication of sharp silicon tips is proposed in the applications of scanning probe microscope (SPM) and vacuum microelectronics. The formation of this new tip has been explained as the change in the local etch rate brought about by the strong adsorption of surfactant molecules in the apex of a silicon tip. In order to understand this point, the different etch rates of the same orientations tested on the hemispherical silicon (with curvature) and flat samples (no curvature) are investigated, and the adsorption thicknesses (or densities) of surfactants on various silicon surfaces measured by an ellipsometer are obtained. Tips with different aspect ratios (0.8:1 and 6:1) have been successfully fabricated on silicon (1 0 0) and (1 1 1) by the combination of etching in pure tetramethylammonium hydroxide (TMAH) or deep reactive ion etching (DRIE) and etching in surfactant-modified TMAH solutions, having the advantages of good uniformities, low temperatures, low costs and complementary metal oxide semiconductor (CMOS)-compatible. The apex of each resultant tip after the treatment in surfactant-added wet etchants typically can have a radius of curvature of 4–5 nm on silicon (1 0 0) or even ≤2 nm on silicon (1 1 1) without any subsequent oxide sharpening process.