A study on the Low-Temperature etching mechanism of ONO layer components using NF₃/H₂ plasma

The cryogenic etching process is gaining attention as a next-generation technology for channel hole fabrication in 3D NAND flash memory, due to its advantage in forming high aspect ratio (HAR) structures compared to conventional RIE. Prior studies have shown that HF acts as a key etchant at low temperatures, where its enhanced physical adsorption enables HAR etching.

In this study, we examined the etching behavior of SiO₂ and SiN (the main layers of the ONO stack in 3D NAND) using an NF₃/H₂ plasma under cryogenic conditions. QDB simulations, OES, and QMS analyses confirmed that increasing H₂ flow enhances HF generation in plasma.

Under HF dominant conditions, FT-IR analysis showed that SiN forms stable ammonium fluorosilicate such as (NH₄)₂SiF₆ at low temperatures, evidenced by NH peaks. The accumulation of these (NH₄)₂SiF₆ on the surface suppressed the etch rate. In contrast, although NH peaks were also observed on SiO₂. QMS results showed low NH₄F formation, indicating limited (NH₄)₂SiF₆ generation. Instead, the physical adsorption of HF and H₂O played a key role in enhancing the etch reaction, increasing the etch rate.

This study provides insight into SiO₂ and SiN etching mechanisms in NF₃/H₂ cryogenic plasma and supports process optimization for HAR etching in 3D NAND.