Ion-assisted etching and profile development of silicon in molecular and atomic chlorine

Abstract

An ion beam etching study, designed to characterize the important kinetic and transport processes involved in the ion-assisted etching of silicon in both molecular and atomic chlorine, was performed. Monoenergetic argon ions were directed normal to a silicon wafer that was simultaneously exposed to a neutral molecular and/or atomic chlorine beam. Dissociation of the beam was induced by thermally heating the graphite tip of an effusive source via electron impact. Beam composition was characterized using a quadrupole mass spectrometer and was found to be in excellent agreement with a thermodynamic equilibrium model at the source pressure and tip temperature. Unpatterned polysilicon wafers were etched to determine the ion-induced etching yields as a function of ion energy, ion to neutral flux ratio, and neutral flux composition. A physically based kinetic model was developed to represent the yield data, incorporating chlorine adsorption, atomic to molecular chlorine surface recombination, and the ion-induced desorption of adsorbed chlorine and silicon chloride products. Feature profile etching experiments using patterned silicon wafers were also performed under ion and neutral-limited conditions of varying neutral composition. Resulting profiles were examined for aspect ratio dependent etching effects, where traditional lag was observed for features etched using an isotropically distributed background chlorine flux and inverse lag was observed for features etched with a molecular and atomic chlorine flux arriving directly from the effusive source. Microtrenching was also present in the etched features. Computer simulations of the etching process and profile development were performed using the kinetic model and a line-of-sight re-emission model for the chlorine transport. The dependence of the yield on the ion angle of incidence was also incorporated into a simulation for an isotropically distributed molecular chlorine flux and was found to have a significant impact on profile evolution as a function of the ion to neutral flux ratio. Using the simulation, atomic to molecular chlorine recombination effects were also explored as a function of the surface recombination coefficient. Predictions of the simulations were compared to experimentally derived profiles and were found to be in good agreement.