Selective isotropic etching of SiO₂ over Si₃N₄ using NF₃/H₂ remote plasma and methanol vapor

In this study, an isotropic etching process of SiO₂ selective to Si₃N₄ using NF₃/H₂/methanol chemistry was investigated. HF was formed using a NF₃/H₂ remote plasma, and in order to remove the F radicals, which induces spontaneous etching of Si-base material, methanol was injected outside the plasma discharge region. Through this process, etch products were formed on the surface of SiO₂, and then the (NH₄)₂SiF₆ was removed by following heating process. When the H and F radicals were abundant, the highest SiO₂ etch per cycle (EPC) was obtained. And, the increase of H₂ and methanol percentage in the gas chemistry increased the etch selectivity by decreasing the F radicals. The etch products such as (NH₄)₂SiF₆ were formed on the surfaces of SiO₂ and Si₃N₄ during the reaction step and no noticeable spontaneous etching by formation of SiF₄ was observed. By optimized conditions, the etch selectivity of SiO₂ over Si₃N₄ and poly Si higher than 50 and 20, respectively, was obtained while having SiO₂ EPC of ~ 13 nm/cycle. It is believed that the cyclic process using NF₃/H₂ remote plasma and methanol followed by heating can be applied to the selective isotropic SiO₂ etching of next generation 3D device fabrication.