Photochemical etching of silicon: The influence of photogenerated charge carriers

Abstract

Low-intensity cw band-gap excitation enhances the etch rate of silicon by XeF2. It has been proposed that the enhancement mechanism involves participation of photogenerated charge carriers in the fluorination reaction itself. A new study has been made of this system by molecular beam mass spectrometry. The results show that for both n- and p-type silicon the SiF3 free radical is the primary etch product at Ar-ion laser powers exceeding 40 W/cm2. SiF4 is also observed, but its formation is independent of light intensity. The data, including measurements of most probable translational energies, are consistent with a photochemical process being responsible for SiF3 formation. Surface heating, which is minimal, cannot account for the experimental results. Since SiF3 is the principal adsorbate on the surface, it is argued that etching is a result of desorption of SiF3 stimulated by a chemical reaction involving two charge carriers. This is distinct from the photodesorption mechanism usually invoked for semiconductor surfaces, which involves single charge capture by a surface adsorbate. Evidence pertaining to participation of charge carriers in other stages of the fluorination reaction adsorption of XeF2 and diffusion of F- has also been obtained. The data indicate that photogenerated charge carriers inhibit chemisorption of XeF2. Field-assisted diffusion, which has been invoked as a rate-determining process in photoassisted etching of semiconductors, is not found to be so for this system. © 1989 The American Physical Society.