Influence of the primary ion beam parameters (nature, energy, and angle) on the kinetic energy distribution of molecular fragments sputtered from poly(ethylene terephthalate) by kiloelectron volt ions

Abstract

Thin films of poly(ethylene terephthalate) (PET) cast on silicon were bombarded by 2–22 keV In + ions and 12 keV Ga + ions and the positive secondary ions were mass and energy analysed by means of a time-of-flight secondary ion mass spectrometer. Since the secondary ion intensities are seen to be dependent on the primary beam parameters, the purpose of this work was to check if these parameters (mass, primary energy, and incidence angle) also influence the kinetic energy distributions of the sputtered molecular fragment ions. This is done in order to gain a better understanding of the molecular ion emission from polymer targets. In general, the shape of the kinetic energy distribution of PET fragment ions is found to be almost independent upon the primary ion nature and energy, as witnessed by the similar spectra observed with 12 keV Ga + and 7–22 keV In + primaries. This confirms previous studies which indicated that the main parameters governing secondary ion kinetic energies in this energy range were the secondary ion nature and the chemical structure of the organic and polymer targets. Nevertheless, the energy spectra of the fingerprint fragment ions obtained at 2 keV with an impact angle of 65° with respect to the surface normal are broader than those observed for higher energy and lower angle of incidence (∼40°). In comparison with the latter, they exhibit an additional contribution centered around 5–6 eV, i.e. in the high energy tail of the distributions. The integrated intensity of this contribution increases with the fragment size, up to 7% and 15% of the total ion intensity for C 7H 4O + and C 17H 12O 5 +, respectively. The results are discussed in terms of collision cascade propagation in the surface region of the target, by comparing the experimental observations to simulations conducted with the TRIM code. TRIM calculations suggest that the variation of the impact angle is the predominant factor influencing the atom displacements in the first 25 Å below the surface. In the {2 keV, 65°} configuration, the energy transferred to recoils in this region is similar, but the number of interacting atoms is more than two times greater than for the other tested combinations of energy and angle, suggesting an increased probability of collective processes for molecular ion emission.