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

The SiO2 layers, as the dielectric in the multistack storage cells of 3D NAND, are detrimentally dissolved by phosphoric acid (PA) during the selective Si3N4 etching. The thinning of SiO2 layers leads to the failure of subsequent processes; thus, an etching inhibitor for SiO2 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 SiO2 etching by retarding the etch rate over 60% without affecting the Si3N4 etching. The saturation adsorption of UPTES on the SiO2 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 SiO2 surface by Si–O–Si covalent bonds to form ordered UPTES-siloxane layers, avoiding contact between SiO2 and the etchant; whereas oversaturated UPTES polymers physisorb not only on SiO2 but also on Si3N4, impeding the Si3N4 etching. This work contributes to a step forward in the effective wet-etching process in high-density 3D NAND fabrication.