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

• Selective etching of Si 3 N 4 by phosphoric acid is studied with DFT. • H 4 P 2 O 7 is the most reactive etchant in the reaction. • Large reaction heat enables selective etching of Si 3 N 4 over SiO 2 . • Water catalyzes surface reaction by structural proton diffusion. 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 (Si 3 N 4 ) over silicon dioxide (SiO 2 ) surfaces in atomistic level. Ab-initio thermodynamic and kinetic formalisms integrated with density functional theory computation propose that pyrophosphoric acid (H 4 P 2 O 7 ), a condensed form of orthophosphoric acid. (H 3 PO 4 ) at high concentration and temperature, is even more reactive toward Si 3 N 4 than H 3 PO 4 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 H 4 P 2 O 7 is much more exergonic than H 3 PO 4 . Notably, we find that water molecules close to the H 4 P 2 O 7 assist sequential etching process in two ways: catalysis via structural proton transfer, and hydrolysis of diphosphate group in the Si 3 N 4 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 Si 3 N 4 and SiO 2 .