TOF-SIMS STUDY OF ALTERNATE POLYELECTROLYTE THIN-FILMS - CHEMICAL SURFACE CHARACTERIZATION AND MOLECULAR SECONDARY IONS SAMPLING DEPTH

Multilayered assemblies of alternate polyelectrolytes have been synthe sized by dipping charged silicon wafers successively into solutions of polyelectrolytes of opposite charge. In this study, three types of as semblies and several thicknesses are investigated by Time-of-Flight Se condary Ion Mass Spectrometry (ToF-SIMS), in combination with other ch aracterization techniques (X-Ray Photoelectron Spectroscopy (XPS), X-R ay Reflectivity (XRR) and Atomic Force Microscopy (AFM)). The sensitiv ity of ToF-SIMS to the extreme surface provides a powerful tool to ver ify the chemical structure, as well as the spatial homogeneity of the topmost layers. Monolayers of complex polyelectrolytes differing only by the end of the pendant group or by the monomer chain length can be distinguished easily, notwithstanding the interference with the inform ation coming from the underlying layers. The chemical imaging capabili ty of ToF-SIMS allows the identification of the defects and contaminan ts in the surface layer, as well as the verification of the thickness uniformity at a local scale (similar to 1 mu m). In addition, the proo f of a regular build-up is given by the disappearance of the substrate signal (Sif) when the number of layers increases. On the other hand, the question of the information depth in ToF-SIMS, which constitutes a n important issue for the characterization of very thin films, is addr essed. The attenuation depth in the organic film is determined for ato mic and molecular secondary ions (Si+, SiOH+, SiO3H-), mainly by the c orrelation with XPS and XRR data. The decay of the mean emission depth when the ion size increases makes the largest molecular ions the most surface sensitive.