Observation of surface dissociation of low-energy polyatomic ions relevant to plasma processing

To gain insight into surface processes in plasma processing, basic ion beam experiments are performed on two representative polyatomic-ion species: hydrocarbon ions CHx+ for deposition processes and fluorocarbon ions CFx+ for etching processes (x=1,2,…). A single ion species is extracted from an inductive plasma via a mass filter and directed onto aluminum surfaces at energies <150 eV. Significant charge neutralization is observed which leads to backscattering of ∼0.3% CHx+ ions and ∼1% CFx+ ions from the surface bombarded at 100 eV. Most of the ions scattered from the surface have kinetic energies lower than 10 eV. A polyatomic ion impinging on the surface breaks up into smaller ionic fragments. Such surface dissociation is found even at very low incident energies (10–50 eV) in the case of hydrocarbon ions, which is attributed to vibrational excitation of the incident parent ions. In contrast to this, fluorocarbon ions hardly dissociate at such low energies. On the other hand, at high energy (> 100 eV) incidence of both CHx+ and CFx+ species gives rise to dissociation into smaller fragment ions, probably via electronic excitation.