Uniform vertical trench etching on silicon with high aspect ratio by metal-assisted chemical etching using nanoporous catalysts.

Recently, metal-assisted chemical etching (MaCE) has been proposed as a promising wet-etching method for the fabrication of micro- and nanostructures on silicon with low cost. However, uniform vertical trench etching with high aspect ratio is still of great challenge for traditional MaCE. Here we report an innovated MaCE method, which combined the use of a nanoporous gold thin film as the catalyst and a hydrofluoric acid (HF)-hydrogen peroxide (H2O2) mixture solution with a low HF-to-H2O2 concentration ratio (ρ) as the etchant. The reported method successfully fabricated vertical trenches on silicon with a width down to 2 μm and an aspect ratio of 16. The geometry of the trenches was highly uniform throughout the 3D space. The vertical etching direction was favored on both (100)- and (111)-oriented silicon substrates. The reported method was also capable of producing multiple trenches on the same substrate with individually-tunable lateral geometry. An etching mechanism including a through-catalyst mass-transport process and an electropolishing-favored charge-transport process was identified by a comparative study. The novel method fundamentally solves the problems of distortion and random movement of isolated catalysts in MaCE. The results mark a breakthrough in high-quality silicon trench-etching technology with a cost of more than 2 orders of magnitude lower than that of the currently available methods.