Understanding of Etching Mechanism of Si₃N₄Film in H₃PO₄Solution For The Fabrication of 3D NAND Devices

The etching mechanism of the Si3N4 in H3PO4 solution is proposed. The effect of H3PO4 concentrations on Si3N4 etching rate is investigated. The Si3N4 etching rate is not simply proportional to H3PO4. Both H3PO4 and H2O participate in the Si3N4 etching reaction and play their roles. First, H2PO4- which is a weak nucleophile generated from H3PO4 replaces -NH2 on the Si3N4 surface with -H2PO4 by an SN1-like reaction. Next, N-Si which is backbone of Si3N4 network is substituted by H2O with Si-OH by an SN2-like reaction. From the Si3N4 etching mechanism and kinetic studies, it is concluded that the Si3N4 etching rate is proportional to product of concentration of H2O and H3PO4. These results suggest that consumption of H3PO4 can be reduced by using a lower concentration of H3PO4 between two H3PO4 concentrations that can produce a similar Si3N4 etching rates.