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

Abstract 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.