ToF-SIMS study of alternate polyelectrolyte thin films: Chemical surface characterization and molecular secondary ions sampling depth

Multilayered assemblies of alternate polyelectrolytes have been synthesized by dipping charged silicon wafers successively into solutions of polyelectrolytes of opposite charge. In this study, three types of assemblies and several thicknesses are investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), in combination with other characterization techniques (X-Ray Photoelectron Spectroscopy (XPS), X-Ray Reflectivity (XRR) and Atomic Force Microscopy (AFM)). The sensitivity of ToF-SIMS to the extreme surface provides a powerful tool to verify 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 information coming from the underlying layers. The chemical imaging capability of ToF-SIMS allows the identification of the defects and contaminants 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 proof 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 an important issue for the characterization of very thin films, is addressed. The attenuation depth in the organic film is determined for atomic and molecular secondary ions (Si+, SiOH+, SiO3H-), mainly by the correlation 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.