Simultaneous Determination of Thicknesses and Refractive Indices of Ultrathin Films by Multiple Incidence Medium Ellipsometry

摘要

It is shown in this paper that the thickness d_(film) and the refractive index n_(film) of a monolayer film can be determined simultaneously by single wavelength ellipsometry in different ambient media. Self-assembled alkylsiloxane monolayers were adsorbed on silicon from dilute solutions of alkyltrichlorosilane precursors (C_nH_(2n+1)SiCl_3, n = 12, 18) and the ellipsometric phase angle Δ was recorded as a function of time between the start value Δ_(sub) of the blank substrate and the final value Δ_(film) of the complete monolayer film in different solvents. From the obtained phase angle differences Δ_(film) - Δ_(sub) of a particular monolayer film in different solvents, the unknown film parameters d_(film) and n_(film) could be calculated by numerical methods. To optimize the light incidence angle for each particular solvent, a variable angle setup was developed, which required no modifications of the standard instrument optics and utilized a standard spectrophotometric glass cell to hold the sample in contact with the ambient solvent. The two model samples of this study yielded values of d_(film) = 26.4 A and n_(film) = 1.540 for the C_(18) monolayer and d_(film) = 18.8 A and n_(film) = 1.500 for the C_(12) monolayer, showing the expected decrease in film thickness for the sorter-chain compound as well as a concomitant reduction of the film refractive index due to a less ordered, looser packed film structure.