Si3N4 Etching with Carboxylic-Acid-Containing Superheated Water
Authors: Son Changjin , Park Taegun , Kim Taehyeon , Lim Sangwoo 
Published: July 2022

Silicon nitride (Si3N4) has been widely used as an insulating or sacrificial layer in various electronic devices. A removal of Si3N4 is also required in the semiconductor manufacturing process. In particular, in the 3D NAND flash memory manufacturing processes, selective removal of Si3N4 from the Si3N4/SiO2 stack structure is one of the very critical processes. It is known that Si3N4 is etched in high temperature phosphoric acid (H3PO4). Si3N4 is etched by nucleophilic attack of H2PO4
- and H2O in H3PO4 solution [1]. However, as the number of Si3N4/SiO2 stacked layers increases to improve the memory density of 3D NAND, oxide regrowth on the SiO2 layer and non-uniform top-to-bottom etching performance during Si3N4 etching process are the issues that need to be solved [2]. In this study, a novel Si3N4 etching in superheated water is investigated.
As the temperature of H2O increases, the self-ionization of H2O is accelerated, thus the concentrations of H3O+ and OH- ion increase. It was previously reported that Si3N4 is etched by nucleophilic attack of OH- in superheated water [3]. However, in general, superheated water produced a low Si3N4 etching rate with the attack of Si substrate, making it difficult to apply to the Si3N4 etching process. Therefore, in this study, various additives were added to superheated water to selectively etch Si3N4 without material loss of the SiO2 and Si substrate.
LPCVD Si3N4 film was prepared on Si wafer. A patterned Si3N4/SiO2 multi-stack structure was prepared the selective etching of Si3N4. Deionized (DI) water with a resistivity of 18.25 M was used to prepare superheated water in the reactor. Acetic acid, butyric acid, citric acid, formic acid, hexanoic acid, lactic acid or tartaric acid was added to DI water before heating. The etching of process was conducted 160 °C for 20 min. The thickness of the Si3N4 film was measured using a spectroscopic ellipsometry and a field-emission scanning electron microscope.
To increase Si3N4 etching rate and suppress the material loss of the Si substrate during the Si3N4 etching process, various carboxylic acids, which are acidic and nucleophiles, were added to superheated water. The etching rates of Si3N4 in 1 M carboxylic-acid-containing superheated water at 160 °C were shown in Fig. 1(a). To reduce the effect of OH- and investigate the effect of carboxylic acid on Si3N4 etching, HCl was added to superheated water, adjusting the pH of the solution at 1, 2 and 3. Etching rates of Si3N4 are shown in Fig. 1(a). When either of tartaric acid, citric acid, lactic acid, or acetic acid was added to superheated water, Si3N4 etching rate was increased at a given pH, as compared to that obtained in HCl-added superheated water. However, when butyric acid, hexanoic acid or formic acid was added to superheated water, similar or lower etching rates of Si3N4 were observed as compared to the HCl-added superheated water. The results in Fig. 1(a) suggest that Si3N4 etching kinetics differs depending on the type of carboxylic acid added to superheated water.
To investigate the reason for different Si3N4 etching rate with the type of carboxylic acid, the concentrations of carboxylate (RCOO-) ion were calculated from the pK
a values of carboxylic acids at 25 °C. As shown in Fig. 1(b), the etching rates of Si3N4 increased with the concentration of carboxylate ions. Based on these results, it is thought that carboxylate ions play an important role in Si3N4 etching in superheated water. To investigate the selective Si3N4 etching ability of carboxylic-acid-containing superheated water, a Si3N4/SiO2 multi pair-layered structure on Si wafer was etched in tartaric-acid-containing superheated water. The cross-sectional FE-SEM images before and after the Si3N4 etching process were shown in Fig. 2. It is clearly shown that Si3N4 was selectively etched without material loss of SiO2 and Si substrate. Finally, eco-friendly and environmentally friendly carboxylic-acid-containing superheated water process can be a strong candidate that can replace the conventional Si3N4 etching process.
References
[1] T. Park, C. Son, T. Kim, S. Lim, J. Ind. Eng. Chem., 102, 146−154 (2021).
[2] T. Kim, C. Son, T. Park, S. Lim, Microelectron. Eng., 221, 111191 (2020).
[3] C. Son, S. Lim, ECS J. Solid State Sci. Technol., 8, N85−N91 (2019).

Figure 1