Chemical sputtering of fluorinated silicon

Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called "physical sputtering." If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorption of particles, the process could be classified as "chemical sputtering." There are a number of molecules such as C${\mathrm{H}}_4$, C${\mathrm{F}}_4$, C${\mathrm{F}}_3$H, C${\mathrm{F}}_3$Cl, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain adsorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena; however, few systematic quantitative investigations of the subject have been completed. As a prototype system the chemical sputtering of silicon and Si${\mathrm{O}}_2$ under argon-ion bombardment in the presence of a molecular beam of Xe${\mathrm{F}}_2$ has been investigated. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as Si${\mathrm{F}}_4$ (gas) and the rest leaves as silicon atoms or $\mathrm{Si}{\mathrm{F}}_x$ radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of Xe${\mathrm{F}}_2$ flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for Si${\mathrm{O}}_2$ is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, Si${\mathrm{O}}_2$ is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces Si${\mathrm{F}}_4$, and the Si${\mathrm{F}}_4$ is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the Si${\mathrm{F}}_4$ molecule.