Paper PS-FrM9
Model Analysis of the Ion Reflection on Surfaces and the Profile Evolution during Etching of Si in Chlorine- and Bromine-Containing Plasmas

Friday, October 19, 2007, 10:40 am, Room 606

In the profile evolution during plasma etching, the reflection of energetic ions on feature sidewalls is appreciated to be important to cause profile anomalies near the feature bottom such as footing and microtrenching. We have developed a model for the feature profile evolution of Si etching in chlorine- and bromine-containing plasmas. The model incorporates an atomic-scale cellular model of surface reaction layers and Monte Carlo calculation for the trajectory of ions on feature surfaces, including their reflection on and penetration into surfaces. The model takes into account the formation of surface reaction multilayers caused by adsorption of neutrals and penetration of ions, deposition of etch products and by-products, and surface oxidation. This paper presents an investigation of effects of the energetic ion scattering on Si surfaces. In the model, the collision between ions and Si atoms on surfaces is assumed to occur, when the impact parameter is smaller than the cutoff radius; the trajectory of ions is analyzed by the Monte Carlo calculation, based on the momentum and energy conservation for an incident ion through successive two-body elastic collisions with substrate Si atoms. The impact parameter and scattering angle are calculated at each collision in the three-dimensional space. The interaction potential of Si ion is necessary for calculation, and the existing classical potential or Stilling-Weber potential is employed for the potential of Si-Cl. However, the potential of Si-Br is not well known, and so to determine the potential function for Si-Br systems, we perform ab initio quantum chemical calculations based on a density-functional method using Gaussian. Then, we analyze the scattering of Cl⁺ and Br⁺ ions on Si surfaces based on their potential calculated, showing that there is a difference of the scattering on Si surfaces between Cl⁺ and Br⁺ ions. Compared the scattering of Br⁺ ions with that of Cl⁺ ions, the number of Br⁺ ions reflecting from Si substrates is small for all incident angle, the distribution of reflection angles is narrow, and the kinetic energy loss is rather large. The difference of the scattering on Si surfaces is attributed not only to the potential, but also to the masses of atoms. The profile simulation during etching in HBr plasmas, using the potential for Si-Br calculated, is compared with the experiments, to examine the effects of ion reflection on feature surfaces.