TIME-OF-FLIGHT SECONDARY-ION MASS-SPECTROMETRY (TOF-SIMS) STUDY OF SF6 AND SF6-CF4 PLASMA-TREATED LOW-DENSITY POLYETHYLENE FILMS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to m onitor in a semi-quantitative way the chemical modifications produced at low-density polyethylene (LDPE) film surfaces by SF6 and SF6-CF4 pl asma treatments. The influence of the plasma treatment time (SF6 plasm a) and the gas composition (SF6-CF4 plasma) have been investigated, Th e ToF-SIMS results are compared with plasma diagnostics (mass spectrom etry and optical emission spectroscopy) and with some earlier XPS char acterization. The ToF-SIMS study of SF6 plasma-treated LDPE films as a function of the treatment time shows two different structural develop ments, Beside the expected formation of CxFy structures due to the gra fting of F atoms, ToF-SIMS detects the presence of SFx species at the surface. Their high sensitivity to ablation, due to the plasma ions, w as related to two possible interactions of plasma species with the LDP E surface: an adsorption of SFx+/- ions or a dissociative chemisorptio n of SFx neutral species. In the latter case, the low binding energy o f the S-C bond can explain the high sensitivity to ablation. In the ca se of SF6-CF4 plasma treatment, the incorporation of fluorine, as seen by ToF-SIMS, is a function of the gas composition. Two maxims are obs erved when the second gas (either SF6 or CF4) is introduced at low con centration (similar to 10%). These maxims are confirmed by XPS and act inometric results and are explained with respect to the reciprocal inf luence of both gases in the plasma discharge.