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

Abstract

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 SN1-like process composed of an NH2-leaving step from Si and an H2PO4-coupling step. Next, an SN2-like reaction occurs, in which N-Si of N-Si-H2PO4 is broken by H2O and replaced with Si-OH. The latter SN2-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 SN2-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.