Development of a Highly Selective Etching Process for SiO2 over Si and SiNx Using F/H-Based Remote Plasmas and a Vapor Phase Solvent

Isotropic etching is traditionally performed using liquid-based wet etching techniques. However, with the increasing integration of devices, achieving conformal etching in high aspect ratio patterns becomes difficult, as liquid chemicals are limited in their ability to penetrate deep into these structures. Moreover, during the drying phase following chemical treatment, surface tension can lead to pattern collapse. Consequently, there is a growing need for dry isotropic etching methods to replace conventional wet etching in advanced device manufacturing. In cases where high selectivity for SiO2 isotropic dry etching is required, mixtures of fluorine-based and hydrogen-based gases are typically used to generate HF, which acts as the etchant for SiO2. SiO2 can be etched through two main mechanisms. The first mechanism involves the direct reaction of HF with H2O or alcohol to etch SiO2. The second mechanism involves the formation of (NH4)2SiF6 salt through the reaction of HF with NH3, which occurs in a plasma containing NH3 and NF3. This (NH4)2SiF6 salt is then sublimated and removed in a subsequent heating process at temperatures above 100°C. The reaction of HF with NH3 to form (NH4)2SiF6 can lead to the generation of ammonium salts that may act as contaminants within the etching chamber. Additionally, this approach necessitates an extra thermal treatment step to eliminate the (NH4)2SiF6 salt. HF can be supplied either directly as vapor or generated through plasma containing fluorine and hydrogen. Plasma-based methods generally offer advantages in terms of safety and process efficiency compared to direct HF vapor usage In this study, we investigated an isotropic dry etching process in which HF etchant is generated by discharging fluorine-based and hydrogen-based gas mixtures. A vapor-phase solvent is supplied outside the discharge region to ionize HF and directly etch SiO2. When using H2O vapor as the solvent, we achieved the etch selectivity of SiO2 over SiNx and poly Si higher than 100 and 600, respectively, was obtained while having SiO2 EPC of ~30 nm/min by optimized conditions. The etching mechanism was clarified through gas-phase and surface analyses using optical emission spectroscopy (OES), quadrupole mass spectrometry (QMS), and X-ray photoelectron spectroscopy (XPS). To assess pattern collapse, we compared the wet phase and vapor phase processes applied to the Si trench pattern. The results suggest that using fluorine and hydrogen-based remote plasmas with a vapor-phase solvent is promising for highly selective isotropic SiO2 etching in the fabrication of next-generation 3D devices.