Semiempirical profile simulation of aluminum etching in a Cl2/BCl3 plasma

Abstract

A semiempirical profile simulator to predict topographic evolution during Cl2/BCl3 plasma etching of photoresist patterned Al lines has been developed. Given incident flux distributions, the profile simulator uses a combination of a particle based Monte Carlo algorithm and analytic ray-tracing algorithm for solving feature-scale ion and neutral flux transport, respectively. We use angular and energy distributions for reflected ions that are consistent with experimental observation and molecular dynamic simulations. Etch yields with energy and angular dependence are experimentally determined for physical sputtering and ion-enhanced etching. The spontaneous etch rate of A1 by chlorine and the spontaneous desorption rate of Cl from photoresist are estimated from experimental results. Sticking coefficients for etchant, chlorine, and depositor, CClx, and depositing flux are determined by fitting simulated profiles to experimental data. A semiempirical site-balance model is developed to compute the surface coverage of etchant. The reaction probability of neutrals at surfaces is self-consistently determined from the surface coverage at incident location. Competition between etching and deposition on feature sidewalls is modeled. A shock-tracking method is used to advance the profile using computed etch/deposition rates. Simulation results are presented which demonstrate that facet formation, aspect ratio dependent etching, and critical dimension gain, are captured accurately by the calibrated profile simulator. In addition, the calibrated profile simulator along with results of a 23 design of experiments in which photoresist and Al etch rates were measured on open frame wafers have been used to create a plasma reactor model. The reactor model relates the operational parameters including inductively coupled power, rf bias and gas flow ratio to the flux variables, i.e., Cl flux, ion flux, ion energy, and depositor flux, that are used as inputs to the profile simulator. In this manner, calibration of the profile simulator requires a minimum of high resolution, expensive, patterned wafers. The reactor model so created is shown to be in quantitative agreement with results from the hybrid plasma equipment model (Ref. 1).