Understanding of Etching Mechanism of Si3N4 Film in H3PO4 Solution For The Fabrication of 3D NAND Devices
Authors: Park Taegun , Son Changjin , Kim Taehyeon , Lim Sangwoo 
Published: July 2022

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 SN1-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 SN2-like reaction of H2O. This SN2-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).

Figure 1