Microfabrication of miniature aperture at the apex of SiO₂ tip on silicon cantilever for near-field scanning optical microscopy

In this paper we present a novel batch fabrication method of a miniature aperture at the apex of SiO₂ tip on a Si cantilever for near-field scanning optical microscopy (NSOM). The Si cantilever was microfabricated from Si(100) wafer, a SiO₂ pyramidal tip was formed near the end of the Si cantilever by thermal oxidation at low temperature. Due to the effect of locally compressive intrinsic stress, the thermal silicon oxide at the apex is thinner than that at the side wall of the pyramidal etch pit. Thus, a tiny aperture can be created at the apex of SiO₂ tip by selective etching the SiO₂ in buffered-HF (BHF) solution. Using this etching method aperture size ranging from 150 to 500 nm have been successfully fabricated. By optimizing the SiO₂ etching time and oxidation condition, sub-100 nm size aperture or aperture array are feasible using the fabrication method. The fabricated probe was combined to an atomic force microscope (AFM) as an AFM/NSOM probe. The AFM in contact mode and corresponding NSOM images of several surfaces were simultaneously obtained. By optimizing the structure of the cantilever we expect to produce a high frequency probe for NSOM, NSOM based data storage and lithography.