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 SiNx, 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 SiNx from ∼35 to ∼50 compared to continuous wave (CW) plasma. The improvement of the SiO2 etch rate and the selectivity of SiO2 over SiNx 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 SiNx 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. Thus, the application of pulsed plasma not only increases SiO2 etching but also enhances the etch selectivity over SiNx.