Experimental and Machine Learning Analysis of Low-Temperature NF3/H2 Plasma Etching of SiO2 and SiN

Abstract

Three-dimensional (3D) NAND flash memory has adopted a vertical stacking structure to overcome the scaling limitations of two-dimensional (2D) NAND memory cell, and the number of oxide-nitride- oxide (ONO) layers continues to increase in pursuit of higher integration density. In April 2024, Samsung commenced mass production of its 290-layer 3D NAND, while SK Hynix introduced a 321- layer device in November of the same year. Furthermore, Samsung has set the ambitious target of developing 1,000-layer 3D NAND by 2030. With this trend, high-aspect-ratio (HAR) channel hole etching has become indispensable. However, as the channel depth increases, conventional reactive ion etching (RIE) processes face inherent limitations, for which cryogenic etching has been proposed as a promising alternative due to its superior capability in etching high-aspect-ratio (HAR) structures. In cryogenic etching, hydrogen fluoride (HF) is known to undergo physical adsorption on SiO₂ and SiN surfaces under cryogenic conditions. Through surface diffusion mechanisms, the adsorbed species are able to reach the bottom of the etching profile, thereby enabling HAR structure formation. Numerous studies have experimentally reported the physical adsorption behavior of HF on SiO₂ and SiN surfaces. Accordingly with the background, the objective of this study is to elucidate the etching mechanisms of SiO₂ and SiN, the key materials of the ONO stack, composed of SiO2 and SiN stack, under NF₃/H₂- based cryogenic plasma conditions, with particular focus on the role of HF adsorption. To investigate the cryogenic etching mechanism, a CCP-RIE system was employed as the primary etching tool, accompanied by plasma diagnostics using Optical Emission Spectroscopy(OES) and Quadrupole Mass Spectrometry (QMS), surface characterization via FT-IR and XPS, and complementary analysis through zero-dimensional plasma simulations. This comprehensive approach provided both experimental and simulation-based methodologies for investigating plasma reactions and surface phenomena under cryogenic conditions. (Chapter 2) The experimental results of SiO₂ and SiN etching in NF₃/H₂ plasmas are presented (Chapter 3). Distinct etching behaviors were observed between the two materials. For SiN, plasma diagnostics revealed that increasing the H₂ flow rate enhanced HF generation within the plasma, a trend that was quantitatively validated by simulation results. FT-IR spectra exhibiting NH₄-related peaks (~3330 cm⁻¹) confirmed the formation of (NH₄)₂SiF₆ on SiN surfaces. The intensity of these peaks increased at lower substrate temperatures, indicating the stabilization of (NH₄)₂SiF₆ and its role in suppressing SiN etching. In contrast, although (NH₄)₂SiF₆ signals were also detected on SiO₂ surfaces, the presence of H₂O — produced as a reaction by-product — weakened the bonding energy of the fluoro-silicate compounds, thereby preventing etch inhibition. Instead, the physical adsorption of HF and H₂O was identified as the dominant factor contributing to the enhanced etch rate of SiO₂. Furthermore, the influence of various experimental parameters as well as the OES and QMS measurements on the etching results was quantitatively evaluated using a machine-learning-based Shapley Additive exPlanations (SHAP) analysis. The SHAP analysis consistently identified the substrate temperature and HF generation density as key factors governing the etching performance. In conclusion, this study elucidated the contrasting etching mechanisms of SiO2 and SiN under NF3/H2-based cryogenic plasma conditions. By integrating experimental diagnostics with machine- learning-driven SHAP interpretation, the underlying cryogenic etching mechanisms were quantitatively understood. The findings provide fundamental insights essential for optimizing cryogenic etching processes for HAR channel hole fabrication in 3D NAND and are expected to contribute to productivity improvements in next-generation high-density 3D NAND manufacturing as well as the broader application of cryogenic plasma technologies.