Unraveling the selective etching mechanism of silicon nitride over silicon dioxide by phosphoric acid: First-principles study

Highly selective etching of a target silicon compound is essential in semiconductor fabrication. Searching for alternatives to conventional etchant that contain hazardous chemicals is challenging, largely due to the unclear understanding of the chemical reaction. In this study, we elucidate etching machinery of phosphoric acid and its outstanding selectivity toward silicon nitride (Si₃N₄) over silicon dioxide (SiO₂) surfaces in atomistic level. Ab-initio thermodynamic and kinetic formalisms integrated with density functional theory computation propose that pyrophosphoric acid (H₄P₂O₇), a condensed form of orthophosphoric acid. (H₃PO₄) at high concentration and temperature, is even more reactive toward Si₃N₄ than H₃PO₄ which has been regarded as a dominant species for decades. We demonstrate that the superior etching selectivity derives from reaction mechanism of thermodynamic control, where H₄P₂O₇ is much more exergonic than H₃PO₄. Notably, we find that water molecules close to the H₄P₂O₇ assist sequential etching process in two ways: catalysis via structural proton transfer, and hydrolysis of diphosphate group in the Si₃N₄ surface. Our study guides a quick and accurate screening as well as designing efficient and safe etchants, which facilitates the fabrication of nanoscale semiconductor devices that accompanies selective etching of alternately stacked hundreds of atomic layers of Si₃N₄ and SiO₂.