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

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