Selective Etching of SiO₂ by Radical Recombination through NF₃/H₂ Pulsed RF Plasma

The structure of semiconductor devices is changing from a two-dimensional (2D) structure to a three-dimensional (3D) structure. For the processing of these 3D devices, the development of selective isotropic etching techniques is required. In this study, the isotropic cyclic dry etching of SiO₂ using NF₃/H₂ plasma for HF generation and NH₃ gas as a solvent has been investigated. Especially, the effects of pulsed radio frequency (RF) plasma on the control of the etch species and the etch characteristics of SiO₂, such as the etch rate and selectivity over SiN_x, have been investigated. The application of pulsed plasma with a duty ratio of 50% improved the SiO₂ etch depth per cycle (EPC) to ∼20% and improved the etch selectivity over SiN_x from ∼35 to ∼50 compared to continuous wave (CW) plasma. The improvement of the SiO₂ etch rate and the selectivity of SiO₂ over SiN_x through the application of pulsed plasma is attributed to the recombination of species during the RF-off period. During the RF-off period, HF reacts with NH₃ introduced into the process chamber to produce NH₄F. Additionally, the improved etch selectivity over SiN_x is due to the removal of F radicals during the RF-off period, achieved by the recombination of F with H and subsequent reaction with NH₃.