Etch characteristics of maskless oxide/nitride/oxide/nitride (ONON) stacked structure using C4H2F6-based gas

Oxide/Nitride/Oxide/Nitride (ONON; SiO₂/SiN_x/SiO₂/SiN_x) stacked structure is widely used in the 3D vertical structure of semiconductor cells. Previously, to form a 3D cells, photoresist (PR) was patterned and repeatedly trimmed on the top of ONON after the etching of one ON layer. Due to the time-consuming process of etching layer-by-layer of ON layer, two-step etch processing using C₄F₈-based or C₄F₆-based gases composed of maskless ONON stack feature etching and followed one ON layer-by layer etching by PR trimming in the ONON stack feature are employed these days. However, the two-step etching method resulted in poor etch profiles of maskless ONON stack feature in addition to high global warming potential of C₄F₈ and C₄F₆. In this study, we investigated the etching of maskless ONON stack feature using C₄H₂F₆-based gas having a low global warming potential and the effects of C₄H₂F₆-based gas on the etch characteristics of maskless ONON stack feature such as etch rate, etch profile, change in critical dimensional (CD), and etch selectivity between SiO₂ and SiN_x have been investigated. C₄H₂F₆-based gas showed the highest etch rates compared to C₄F₆ and C₄F₈-based gases in addition to the etch selectivity of ~ 1:1 between SiO₂ and SiN_x due to hydrogen included in the gas structure. In addition, the change in horizontal CD was lower in the order of C₄H₂F₆, C₄F₆, and C₄F₈-based gases due to the more effective sidewall passivation in the order of C₄F₈, C₄F₆, and C₄H₂F₆-based gases. The thicker carbon-based polymer layer on the sidewall also played an important role in maintaining the shape of the top edge shape of maskless ONON stack feature when etching a line feature in an environment without a mask.