Silicon nitride (Si₃N₄) has been widely used as an insulating or etch stop layer in the semiconductor devices [1]. For example, in a 3D V-NAND flash memory manufacturing process, Si₃N₄ is alternately stacked with silicon oxide (SiO₂) to form a NAND structure. Here, it is necessary to selectively etch Si₃N₄ avoiding etching of SiO₂ in the repeated Si₃N₄/SiO₂ multi-stack layers. In general, phosphoric acid (H₃PO₄) is used to selectively etch Si₃N₄ against SiO₂ [2]. However, since the concentration of H₃PO₄ solution changes with the evaporation of H₂O, it is not easy to determine the concentration dependence of Si₃N₄ etching rate in H₃PO₄ solution. In order to properly control the shape of the 3D V-NAND structure, it is necessary to extensively understand the effect of H₃PO₄ concentration on the Si₃N₄ etching. The study of Si₃N₄ etching has been mainly focused on the effect of additives in H₃PO₄ on the behavior of Si₃N₄ etching to solve the issue such as oxide regrowth so far [3, 4]. Fundamental study of the Si₃N₄ etching reaction mechanism in H₃PO₄ is lacking. In this study, the Si₃N₄ etching mechanism in H₃PO₄ solution is elucidated systematically.

For the investigation of Si₃N₄ etching reaction mechanism, LPCVD Si₃N₄ blanket wafer and Si₃N₄/SiO₂ multi-stack layered trench wafer were used. Etching experiments of of Si₃N₄ were conducted in 10-95 wt% H₃PO₄ solutions at 160 °C. Spectroscopic ellipsometry and field emission scanning electron microscopy were used to measure the etching rate of Si₃N₄ after etching process.

First, the changes in etching rate of Si₃N₄ in the various concentrations of H₃PO₄ solution were investigated. As shown in Fig. 1, according to our kinetic calculation and experiments, etching rate of Si₃N₄ increased as the H₃PO₄ concentration increased until the concentration of H₃PO₄ reaches a certain concentration that produced the highest etching rate. However, etching rate of Si₃N₄ decreased with the concentration of H₃PO₄ beyond the critical H₃PO₄ concentration. According to the results, there may be two concentrations of H₃PO₄ solution which produce an identical etching rate of Si₃N₄. It is also suggested that not only H₃PO₄ but also H₂O plays an important role in the Si₃N₄ etching kinetics.

To investigate the role of H₂O and H₃PO₄ in the etching reaction of Si₃N₄, kinetic isotope effect (KIE) of Si₃N₄ etching reaction in H₃PO₄ solution was used. KIE provides information about which reactant determines the rate-limiting step. In this study, each reactant of Si₃N₄ etching reaction, H₂O or H₃PO₄, was substituted with D₂O and D₃PO₄, respectively. Then, etching of Si₃N₄ was conducted in four different solutions. As shown in Fig. 2, when H₃PO₄ was replaced by D₃PO₄, the Si₃N₄ etching rate was not significantly changed. However, when H₂O was replaced by D₂O, the Si₃N₄ etching rate decreased by 30 %. Therefore, it is thought that rate-limiting step of the Si₃N₄ etching reaction is determined by an elementary reaction related to H₂O rather than H₃PO₄.

Based on the above results, Si₃N₄ etching reaction mechanism was suggested. First, the Si₃N₄ surface termination (-NH₂) is substituted with H₂PO₄^-, which is a weak nucleophile, by an S_N1-like reaction. When H₂PO₄^- binds to Si, the backbone of Si₃N₄ is weakened because the electronegativity of O (3.44) is greater than N (3.04). Next, the Si-N of the weakened backbone of Si₃N₄ is substituted with Si-OH by an S_N2-like reaction of H₂O. This S_N2-like reaction is considered as a rate-limiting step. It is believed that understanding of etching mechanism of Si₃N₄ in H₃PO₄ solution will improve selective etching process of Si₃N₄ for the integration of 3D V-NAND.

References

[1] A. Grill, P.R. Aron, Thin Solid Films, 96, 25-30 (1982).

[2] W.V. Gelder, V.E. Hauser, J. Electrochem. Soc., 114, 869-872 (1967).

[3] D. Seo, J. Bae, E. Oh, S. Kim, S. Lim, Microelectron. Eng., 118, 66-71 (2014).

[4] T. Kim, C. Son, T. Park, S. Lim, Microelectron. Eng., 221, 111191 (2020).