Paper SS+AS+HC-FrM7
Quantum Mechanics and Reaction Kinetics Study on SiO₂ and SiN Dry Isotropic Chemical Etching Process

Friday, October 26, 2018, 10:20 am, Room 203C

Dry isotropic chemical etching processes are important for semiconductor manufacturing, but such processes often require subtle process tuning to achieve high etching rates and the desired etching selectivity between SiO₂ and SiN. For example, the dry chemical etching solely with HF gas (Process 1) requires fine tuning of conditions for SiN etching rate because it has a peaky dependence on the process temperature; whereas dry chemical etching with NH₃/HF binary gas mixtures (Process 2) requires subtle tuning to simultaneously maximize etching rate and SiO₂:SiN selectivity. Notably, in Process 2, SiN etching rate increases with the etching time while SiO₂ etching rate slows down with the etching time. This slowdown is attributed to the formation of an etchant diffusion barrier from the solid byproduct, AFS (Ammonium fluorosilicate). Because of these contradictions, it has been difficult to achieve highly selective and rapid SiO₂ etching, thus a better understanding of the etching mechanisms is important to further develop the high selectivity required for the formation of scaled multicomponent semiconductor device structures. This study therefore focuses on revealing these mechanisms by using the quantum mechanics and by the analysis of reaction kinetics.

Firstly, Process 1 was studied with a quantum mechanical analysis by using the GRRM (Global Reaction Route Mapping) program. For this study, GRRM searched possible etching reaction paths automatically. From this reaction path search, it was revealed that SiN etching by HF gas, when used by itself, is much more rapid than SiO₂ etching. Further analysis was conducted by the reaction kinetics analysis. The kinetics parameters comprised HF adsorption, desorption and etching reactions. This model shows good agreement with the experimental SiN etching behavior.

Moreover, Process 2 was studied with similar quantum mechanics and kinetics analyses. Quantum mechanics analysis revealed that NH₃ combined with HF enhances both SiO₂ and SiN etching reactions. An NH₄F etching model was then added to the kinetics model. Because the etching byproduct, AFS, may be both an SiN etching accelerator and an SiO₂ etching decelerator, we could model the SiN etching kinetics initiated by NH₄F and propagated by NH₄F + AFS. This model quantitatively agrees with the experimental SiN etching data. Likewise, the SiO₂ kinetics model is composed of NH₄F etching, the diffusion resistance through solid AFS and the sublimation of AFS. This model also shows good agreement with experiment.