Photochemical etching of silicon: The influence of photogenerated charge carriers

Low-intensity cw band-gap excitation enhances the etch rate of silicon by ${\mathrm{XeF}}_2$. 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 ${\mathrm{SiF}}_3$ free radical is the primary etch product at Ar-ion laser powers exceeding 40 W/${\mathrm{cm}}^2$. ${\mathrm{SiF}}_4$ 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 ${\mathrm{SiF}}_3$ formation. Surface heating, which is minimal, cannot account for the experimental results. Since ${\mathrm{SiF}}_3$ is the principal adsorbate on the surface, it is argued that etching is a result of desorption of ${\mathrm{SiF}}_3$ 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 ${\mathrm{XeF}}_2$ and diffusion of ${\mathrm{F}}^{\mathrm{-}}$—has also been obtained. The data indicate that photogenerated charge carriers inhibit chemisorption of ${\mathrm{XeF}}_2$. 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.