Morphology and Chemical Termination of HF-Etched Si₃N₄ Surfaces

Several reports on the chemical termination of silicon nitride films after HF etching, an important process in the microelectronics industry, are inconsistent claiming N-Hx, Si-H, or fluorine termination. An investigation combining infrared and x-ray photoelectron spectroscopies with atomic force and scanning electron microscopy imaging reveals that under some processing conditions, salt microcrystals are formed and stabilized on the surface, resulting from products of Si₃N₄ etching. Rinsing in deionized water immediately after HF etching for at least 30s avoids such deposition and yields a smooth surface without evidence of Si-H termination. Instead, fluorine and oxygen are found to terminate a sizeable fraction of the surface in the form of Si-F and possibly Si-OH bonds. The fluorine termination is remarkably stable in air and water vapor in ambient conditions, with clear implications on further surface chemical functionalization.