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

Abstract

It is shown in this paper that the thickness dfilm and the refractive index nfilm 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 (CnH2n+1SiCl3, n = 12, 18) and the ellipsometric phase angle Δ was recorded as a function of time between the start value Δsub of the blank substrate and the final value Δfilm of the complete monolayer film in different solvents. From the obtained phase angle differences Δfilm − Δsub of a particular monolayer film in different solvents, the unknown film parameters dfilm and nfilm 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 dfilm = 26.4 Å and nfilm = 1.540 for the C18 monolayer and dfilm = 18.8 Å and nfilm = 1.500 for the C12 monolayer, showing the expected decrease in film thickness for the shorter-chain compound as well as a concomitant reduction of the film refractive index due to a less ordered, looser packed film structure.