Study of film growth properties of self-assembled polyelectrolyte films of higher thickness: Reflectometric and focused ion beam analyses

This work addresses the use of fixed-angle laser reflectometry to follow in situ the deposition of self-assembled polyelectrolyte films in a large domain of thickness. It is shown theoretically and illustrated experimentally that the reflectometric output can be interpreted in terms of refractive index and thickness way above the usual range which covers the earlier deposition steps of the polymers (i.e., 30-50 layers, depending on the film composition). Thick films containing up to 320 deposition steps (160 bilayers) were made in the reflectometric cell with the polymer pair poly(allylamine, hydrochloride) and poly(styrene sulfonate) on a silica substrate. They were imaged with AFM, and their thickness was measured using the focused ion beam (FIB) technique. In the course of construction, the reflectometric data started to alternate more or less regularly between positive and negative values, exhibiting absolute values at the extrema which decreased progressively. This phenomenon was coherent with simulated data, once the diffusion of the incident beam and the change of refractive index were taken into account. The phenomenon gives an opportunity to evaluate the composition and the mean thickness of the film during the growth, which is confirmed by comparison with the FIB measurements.