Selective Etching\nof SiO<sub>2</sub> by Radical Recombination\nthrough NF<sub>3</sub>/H<sub>2</sub> Pulsed RF Plasma

The\nstructure of semiconductor devices is changing from a two-dimensional\n(2D) structure to a three-dimensional (3D) structure. For the processing\nof these 3D devices, the development of selective isotropic etching\ntechniques is required. In this study, the isotropic cyclic dry etching\nof SiO<sub>2</sub> using NF<sub>3</sub>/H<sub>2</sub> plasma for HF\ngeneration and NH<sub>3</sub> gas as a solvent has been investigated.\nEspecially, the effects of pulsed radio frequency (RF) plasma on the\ncontrol of the etch species and the etch characteristics of SiO<sub>2</sub>, such as the etch rate and selectivity over SiN<sub><i>x</i></sub>, have been investigated. The application of pulsed\nplasma with a duty ratio of 50% improved the SiO<sub>2</sub> etch\ndepth per cycle (EPC) to ∼20% and improved the etch selectivity\nover SiN<sub><i>x</i></sub> from ∼35 to ∼50\ncompared to continuous wave (CW) plasma. The improvement of the SiO<sub>2</sub> etch rate and the selectivity of SiO<sub>2</sub> over SiN<sub><i>x</i></sub> through the application of pulsed plasma\nis attributed to the recombination of species during the RF-off period.\nDuring the RF-off period, HF reacts with NH<sub>3</sub> introduced\ninto the process chamber to produce NH<sub>4</sub>F. Additionally,\nthe improved etch selectivity over SiN<sub><i>x</i></sub> is due to the removal of F radicals during the RF-off period, achieved\nby the recombination of F with H and subsequent reaction with NH<sub>3</sub>. Thus, the application of pulsed plasma not only increases\nSiO<sub>2</sub> etching but also enhances the etch selectivity over\nSiN<sub><i>x</i></sub>.