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.

目标硅化合物的高度选择性蚀刻在半导体制造中至关重要。寻找含有害化学物质的传统蚀刻剂的替代品具有挑战性，这主要是由于对化学反应的了解不清。在这项研究中，我们以原子水平阐明了磷酸的蚀刻机制及其在二氧化硅（SiO ₂）表面上对氮化硅（Si ₃ N ₄）的出色选择性。从头算热力学和动力学形式主义与密度泛函理论计算相结合，提出了焦磷酸（H ₄ P ₂ O ₇），一种正磷酸的缩合形式。

（H ₃ PO ₄）在高浓度和高温下对Si ₃ N _4的反应性甚至比数十年来一直被认为是主要物质的H ₃ PO ₄更具反应性。我们证明了优异的刻蚀选择性源自热力学控制的反应机理，其中H ₄ P ₂ O _7的能效比H ₃ PO ₄高得多。值得注意的是，我们发现水分子接近H ₄ P ₂ O ₇通过两种方式协助顺序蚀刻过程：通过结构质子转移进行催化，以及Si ₃ N ₄表面中二磷酸酯基团的水解。我们的研究指导了快速，准确的筛选以及设计有效且安全的蚀刻剂，这有助于纳米级半导体器件的制造，该器件伴随着对交替堆叠的数百个Si ₃ N ₄和SiO ₂原子层的选择性蚀刻。