Improvement of Si3N4/SiO2 etching selectivity through the passivation of SiO2 surface in aromatic carboxylic acid-added H3PO4 solutions for the 3D NAND integration

Abstract

Selective Si3N4 etching becomes challenging as the number of stacks in three-dimensional Not AND (3D NAND) flash memory devices increases. To suppress changes in the thickness of SiO2 layers in Si3N4/SiO2 multi-stack structures, etching inhibitors should be added to H3PO4 solutions. Notably, carboxylic acids can be adsorbed on OH-terminated substances, such as SiO2, by forming Si-O-C bonds. Therefore, carboxylic acids added to H3PO4 solutions can be adsorbed on SiO2 surfaces, and passivation layers can be formed, suppressing SiO2 etching. When a Si3N4/SiO2 multi-stack structure is dipped in a H3PO4 solution, the Si3N4 layers are removed, and the SiOxNy surfaces on the SiO2 layers are converted into SiOx surfaces with positively charged oxygen vacancies. For aromatic carboxylic acids, C=O bonds in their carboxyl groups can be converted to C-O-, causing electrostatic attraction between the C-O- groups and oxygen vacancies on SiOx surfaces. Consequently, aromatic carboxylic acids can be adsorbed on SiOx surfaces via the electrostatic attraction and Si-O-C covalent bond formation, creating robust passivation layers and suppressing SiO2 etching. However, aliphatic carboxylic acids form only Si-O-C bonds on SiOx surfaces and hence cannot realize the same functions as their aromatic counterparts.