Chemical modification of subcritical water for the dissolution of Si₃N₄ layer in the fabrication of microelectronic devices

Subcritical water was used to selectively remove Si₃N₄ layers in the 3D NAND fabrication. Various fluorine compounds were added to subcritical water. When sulfonyl fluoride-based additives were added to subcritical water, the dissolution of Si₃N₄ was accelerated by F^-, and the Si₃N₄-to-SiO₂ removal selectivity increased owing to the high [F^-]/[HF₂^-] ratio and low OH^- concentration. Furthermore, the addition of H₂SiO₃ to the subcritical water resulted in selective and uniform Si₃N₄ removal without the loss of SiO₂ layers in the 128-layer Si₃N₄/SiO₂ stack structure. However, the addition of ammonium fluoride compounds to subcritical water could not achieve selective removal of Si₃N₄. The addition of HCl to the ammonium fluoride-containing subcritical water suppressed the dissolution of Si and SiO₂. Further addition of H₂SiO₃ and optimization of the additive concentrations led to selective and uniform Si₃N₄ removal in the 128-layer Si₃N₄/SiO₂ stack structure without the loss of the Si substrate and SiO₂ layers.