Predictive modeling of feature profile evolution is critical to understand the complex plasma-surface interactions and aid the design of etch chemistry and development of etch systems, especially when the feature dimension is getting much smaller and design of experiments is getting much more costly. In this work, a direct simulation Monte Carlo (DSMC) method is used to develop a profile simulator that predicts the profile evolution during the shallow trench isolation etch (STIE) in chlorine-based plasmas. Due to the discretized cells used in the simulation, a segmented surface advancement algorithm was developed to accurately define the surface normal, which dictates particle scattering upon collision. Plasma parameters such as neutral density, ion density, and mean ion energy are determined from a reactor scale model, while ion energy distribution function (IEDF) and ion angular distribution function (IADF) are verified by a particle-in-cell (PIC) model.¹

A fractional factorial design of experiments was carried out to determine the major processing parameters affecting the feature profile evolution in a Cl₂, O₂, and N₂ plasma in a dual coil inductively coupled plasma reactor. Out of seven parameters investigated, the chamber pressure and DC ratio (ratio of currents to the inner and outer coils) were found to have the most effect on the feature side wall angle (SWA), one of the most critical parameters defining the success of STIE. Additional experiments were performed to assess the effect of O₂ addition as well as to ascertain the effects of source power and DC ratio. Scanning electron microscopy (SEM) was used to assess the feature profile evolution in various plasma chemistries under different etch conditions. Slight microtrenching was observed in features etched by pure chlorine (no O₂) and was accurately predicted by the profile simulator, sampling only ions (Cl⁺), neutral etchants (Cl), and a minute ratio of neutral passivants (e.g. etch byproduct, SiCl ₂). A change in the plasma density and substrate bias (translating to ion energy) were found to significantly alter the sidewall tapering as well as the etch feature depth. The profile simulator was able to capture these changes in SWA and etch depths by properly determining the full-width half maximum of the ion angular distribution (IAD) as a function of substrate bias and adjusting the neutral to ion ratio which is affected by the pressure and source power. The erosion of the hard mask during STIE was found to occur in a high density plasma under high substrate bias, affirming an etching threshold energy of the hard mask, and the simulator was successful in predicting the double facets on the hard mask and the resulting feature profile.

¹ A. C. F. Wu, M. A. Lieberman, and J. P. Verboncoeur, Journal of Applied Physics 101 (2007)