Surface reactions and optical diagnostics of SiNx:H etching processes at cryogenic temperatures in CF4/H2 plasma mixtures

Etching mechanisms of plasma enhanced chemical vapor deposited silicon nitride (SiN) using CF4/H2 plasmas with various substrate temperature, Ts, and substrate bias were investigated in situ using Fourier transform infrared spectroscopy, ellipsometry and optical actinometry. Surface characterization revealed N-H and NH4 absorbance bands when Ts was cooled to −60 °C, indicating ammonium fluorosilicate (AFS) formation, and subsequent etch rates dictated by applied bias voltage led to a proposed etching scheme by CHxFy and ion species. Consequently, using high bias voltage, a 40% etch rate enhancement was achieved at Ts = −60 °C and 70%-H2 compared to ambient temperature, coinciding with increased NH density from increased reactions between ions and AFS. Meanwhile, the ongoing presence of CN radicals signified persistence of the main etch contributor, CHxFy. Conversely, the etch rate dependence with Ts was not reflected by F density trends, while that of H hinted at increased HF adsorption with deeper cooling. Nonetheless, a reduction in etch rate and F density with lower CF4 content, regardless of temperature, highlighted the necessity of CHxFy and F species. Consequently, both high bias voltage and cryogenic temperatures are key to exploiting the AFS layer for enhanced etch rates and achieving rapid 3D NAND production.