Molecular-dynamics model of energetic fluorocarbon-ion bombardment on SiO2. II. CFx+ (x=1, 2, 3) ion etch characterization

A molecular-dynamics-based model has been used to understand etching of SiO2, with and without a fluorocarbon-polymer layer, by energetic fluorocarbon (CFx+) ions. The test structures for computational experiments are prepared by starting with α-quartz ([001] orientation) and bombarding it with low-energy ions: Ar+ ion for amorphous and fluorocarbon ions for fluorocarbon-polymerized test structures. CF+, CF2+, and CF3+ ions with a range of energies and angles of impact are then bombarded on these test structures to characterize fluorocarbon-ion etching. Results show that aggregate Si and O etch yields increase with ion energy for all ions and all angles of impact. Etch yields, however, exhibit nonlinear dependence on angle of impact with a peak around 60°. This nonlinear behavior is attributed to the balance among the incident ion energy transfer fraction, depth of energy deposition, and cluster scattering direction during secondary scattering events. The Si etch yield increases going from CF+ to CF2+ and then decreases for CF3+. This etch yield dependence on the nature of ion is because the amount of F per ion increases but the energy per F atom decreases as one moves from CF+→CF2+→CF3+. Si and O etch yields decrease considerably without the presence of the fluorocarbon layer, emphasizing the importance of C and F in etching SiO2. Parent ion clusters are only observed in the etch effluent at low ion energy and near the grazing angle of incidence. Under other conditions, the incident ion fragments upon impact at the surface. If fragments of the incident ions make it out of the surface, they retain a substantial fraction of the incident ion energy and their angle of ejection from the surface is related to the ion angle of incidence. However, clusters that contain test structure atoms are ejected from the surface with only a few eV of energy, and their angular and energy distributions are relatively independent of incident ion characteristics.