Sub-100-nm-wide, deep trenches defined by reactive ion etching

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Abstract
In this study, reactive ion etching (RIE) has been used to define vertical trenches with dimensions considerably smaller than the lithographically defined width. Gratings with 320 nm period were defined using a masked ion beam exposure of PMMA, followed by lift-off of a Ni mask. For shallow etching, tapered profiles were obtained. For more deeply etched structures, the tapered walls converged and formed sub-100-nm-wide, highly anisotropic trenches. The trench formation is not a crystal orientation dependent effect and has been obtained in GaAs, SiO2, and nonstoichiometric silicon nitride (SiNx). The etched profiles and the final trench widths depend on the RIE conditions and the substrate materials. Ion divergence in the RIE reactor was measured using nitrocellulose and was found to increase with chamber pressure. The measured half-angle of ion divergence was 0.7° at 2 mTorr and increased to 1.2° at 20 mTorr during RIE in CF4 at 500 V self-induced dc bias voltage. The ion distribution inside these trenches was calculated using a model based on the Monte Carlo method. The result indicated that ion flux decreased at the bottom of the trenches, but increased near the top of the trenches due to ion scattering from the tapered sidewalls. These ion scattering effects contribute to the formation of trenches ranging from 20 to 60 nm in width and as deep as 1 μm.

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