Study on the SiO₂ Wet-Etching Mechanism Using γ-Ureidopropyltriethoxysilane as an Inhibitor for 3D NAND Fabrication

The SiO 2 layers, as the dielectric in the multistack storage cells of 3D NAND, are detrimentally dissolved by phosphoric acid (PA) during the selective Si 3 N 4 etching. The thinning of SiO 2 layers leads to the failure of subsequent processes; thus, an etching inhibitor for SiO 2 is crucial for 3D NAND fabrication. However, most inhibitors are currently limited to neutral or alkaline solutions. In this work, the γ-ureidopropyltriethoxysilane (UPTES), compatible with a hot-concentrated PA solution, was applied to the wet-etching process for the first time and exhibited an inhibitory effect on SiO 2 etching by retarding the etch rate over 60% without affecting the Si 3 N 4 etching. The saturation adsorption of UPTES on the SiO 2 substrate is confirmed to occur around the critical concentration, according to the Langmuir–Freundlich adsorption isotherm. The adsorption behavior of UPTES exhibits a reliance on its concentration. Around the critical concentration, the UPTES molecules chemisorb on the SiO 2 surface by Si–O–Si covalent bonds to form ordered UPTES-siloxane layers, avoiding contact between SiO 2 and the etchant; whereas oversaturated UPTES polymers physisorb not only on SiO 2 but also on Si 3 N 4, impeding the Si 3 N 4 etching. This work contributes to a step forward in the effective wet-etching process in high-density 3D NAND fabrication.