Atomistic mechanism for the macroscopic effects induced by small additions of surfactants to alkaline etching solutions

This letter proposes a model to explain the accused macroscopic effects caused by a small addition of a surfactant, such as Triton X-100, to a typical alkaline etchant, such as tetramethyl ammonium hydroxide (TMAH), for applications of silicon micromachining in Micro Electro Mechanical Systems (MEMS). The effects include dramatic changes in the etched profiles and small to large reductions in the etch rate and surface roughness, depending on the surface orientation of silicon and the etchant concentration. We propose that the surfactant forms a thin, adsorbed, filter layer at the silicon surface, reducing the number of reactant molecules that may reach the surface from the etchant phase and affecting the number of performed reactions. The orientation and etchant-concentration dependence of the surfactant layer thickness is determined using spectroscopic ellipsometry. The model is consistent with the most widely accepted mechanism for the etching process and overall observed phenomena in general.