Guided acoustic waves in layered polymer films: interpretation of Brillouin data

We report our theoretical considerations motivated by Brillouin spectra of polymethyl methacrylate (PMMA) films (d = 13 nm and d = 225 nm) on Si(100) substrates. We have rederived a numerical calculational scheme developed fo r considering Brillouin experiments on layer systems. It delivers dispersio n curves and intensities, indicated here by appropriate hatchings. With bac kscattering geometry, we observed guided waves in agreement with our calcul ations. The Brillouin peaks fall on the calculated segments of the dispersi on curves visible with the elasto-optic mechanism for 180 degrees scatterin g or for reflection-induced 'A-scattering' (called RI ThetaA scattering by Kruger et al (Kruger J K, Embs J, Brierley J and Jimenez R 1998 J. Phys. D: Appl. Phys. 31 1913)). The observed pseudo-guided waves for d = 225 nm wer e explained using a model of a PMMA film fixed at one surface and free at t he other. For the d = 225 nm thick film the Brillouin data provided informa tion on its mechanical constants, but in the d = 13 nm case the dispersion curves are not very sensitive to the film properties. The Brillouin data fo r d = 13 run show the direction of the sagittal plane with respect to the s ymmetry of the silicon rather than indicating the properties of the PMMA fi lm.