Green Manufacturing of Silyl-Phosphate for Use in 3D NAND Flash Memory Fabrication

Three-dimensional NAND flash memory featuring dozens of vertically stacked memory cells is the state-of-the-art technology for most storage platforms. To fabricate three-dimensional (3D) NAND memory, lateral etching of the Si₃N₄ layer over SiO₂ is an essential step that is conducted through a wet etching process using a phosphoric acid-based etchant. Silyl-phosphate or highly selective nitride serves as an etching solution additive to control the SiO₂ layer dissolution rate. However, silyl-phosphate is prepared with an expensive monomeric silica precursor and at high reaction temperatures and generates environmentally harmful byproduct gases, such as HCl, HF, and CH₃OH. This study demonstrates that silyl-phosphate can be prepared using low-cost polymeric silica under a mild reaction temperature by changing the characteristic acidity of phosphoric acid. The possibility of tuning the phosphoric acid acidity was first studied by molecular dynamics simulations, and phosphoric acids with stronger acidity were prepared by the evaporation of water from H₃PO₄ (85%). The concentrated phosphoric acid enabled a fast reaction of polymeric silica and phosphate at a low reaction temperature (80 °C). The obtained silyl-phosphate lowered the SiO₂ layer dissolution rate, thereby yielding a Si₃N₄/SiO₂ layer etching ratio of up to 940. The proposed method offers an environmentally friendly production process for special chemicals used in 3D NAND flash memory fabrication.