Understanding of Si₃N₄-H₃PO₄ reaction chemistry for the control of Si₃N₄ dissolution kinetics

This work proposes the mechanism of the silicon nitride (Si₃N₄) dissolution reaction in phosphoric acid (H₃PO₄) solution and shows how to kinetically control the reaction. The various H₃PO₄ concentrations and temperature dependencies of Si₃N₄ dissolution rates and the behavior of Si₃N₄ dissolution with different acids were investigated. First, the dissolution of Si₃N₄ in H₃PO₄ begins with an S_N1-like process composed of an NH₂-leaving step from Si and an H₂PO₄-coupling step. Next, an S_N2-like reaction occurs, in which N-Si of N-Si-H₂PO₄ is broken by H₂O and replaced with Si-OH. The latter S_N2-like reaction is thought to determine the overall dissolution rate of Si₃N₄. When a weak nucleophile, such as H₂PO₄⁻, is coupled with Si, the rate of the S_N2-like reaction increases where the Si-N of the backbone of Si₃N₄ is broken. From the reaction rate based on those reaction mechanisms, the overall dissolution rate of Si₃N₄ is determined by the product of the H₃PO₄ and H₂O concentrations, and that two different concentrations of H₃PO₄ produce identical dissolution rates at a given temperature. It is also indicated that the Si₃N₄ dissolution rate compared to the SiO₂ dissolution rate is higher at a lower concentration of the two H₃PO₄ solutions.