Chemical sputtering of silicon by F+, Cl+, and Br+ ions: Reactive spot model for reactive ion etching

Chemical sputtering yields of silicon resulting from the incidence of isotopically pure 19F+, 35Cl+, and 81Br+ ion beams were measured in the 100--3000 eV energy range. It is found for these cases that the chemical sputtering yields saturate in the high energy range of more than 600 eV. The saturated yields (atoms/ion) are 0.18 for F+/Si, 0.45 for Cl+/Si, and 0.23 for Br+/Si. The yields increased with increasing ion energy in the 100--500 eV region with different slopes. The steepest slope is observed for the case of Br+/Si, and the smallest is the F+/Si case. This difference in the slope indicates that the chemical reaction between Br+ and Si cannot be activated without high acceleration of Br+, and the same occurs for Cl+/Si. On the other hand the F+ ion is assumed to react with Si with a relatively high probability even when the energy is low. On the basis of these results, a reactive spot model is proposed for the ion-assisted chemical etching process. A generalized method to get anisotropic profiles is also presented for reactive ion etching of Si with fluorine-, chlorine-, and bromine-containing gases