Etch characteristics of maskless Oxide/Nitride/Oxide/Nitride (ONON) stacked structure using C4H2F6-based gas

Oxide/Nitride/Oxide/Nitride (ONON; SiO ₂ /SiN _x /SiO ₂ /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 C ₄ F ₈ -based or C ₄ F ₆ -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 C ₄ F ₈ and C ₄ F ₆ . In this study, we investigated the etching of maskless ONON stack feature using C ₄ H ₂ F ₆ -based gas having a low global warming potential and the effects of C ₄ H ₂ F ₆ -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 SiO ₂ and SiN _x have been investigated. C ₄ H ₂ F ₆ -based gas showed the highest etch rates compared to C ₄ F ₆ and C ₄ F ₈ -based gases in addition to the etch selectivity of ~1:1 between SiO ₂ and SiN _x due to hydrogen included in the gas structure. In addition, the change in horizontal CD was lower in the order of C ₄ H ₂ F ₆ , C ₄ F ₆ , and C ₄ F ₈ -based gases due to the more effective sidewall passivation in the order of C ₄ F ₈ , C ₄ F ₆ , and C ₄ H ₂ F ₆ -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.