Simultaneous Determination of Thickness and Index of Refraction of Ultrathin Silane Films by Immersion Ellipsometry, Including Characterization of Perfluorinated Silane Films by Low Energy Ion Scattering

Immersion ellipsometry (IE) was used to simultaneously determine the thicknesses and refractive indices of ultrathin silane films deposited on native oxide-terminated silicon surfaces. Eight structurally distinct silanes, including alkyl, fluorinated, aromatic, and oxygen-containing, were deposited using a liquid-phase sandwich deposition method, producing films with thicknesses in the range of ca. 1.0-2.5 nm. By combining ellipsometric measurements acquired in air and water, the strong correlation between thickness and refractive index that limits conventional ellipsometry for ultrathin films was removed, allowing these parameters to be determined independently and with good reproducibility. The measured refractive indices varied systematically with silane molecular structure and followed expected trends based on chemical composition: the fluorinated silanes exhibited lower refractive indices, aromatic silanes higher values, and alkyl and oxygen-containing silanes intermediate values. X-ray photoelectron spectroscopy (XPS) confirmed the deposition of chemically distinct silane layers, while water contact angle measurements correlated surface wettability with silane chemistry and film thickness. Low-energy ion scattering (LEIS) directly probed the outermost surfaces of fluorinated silane films, enabling quantification of surface coverage and assessment of thermal stability during annealing up to 700 °C. LEIS results showed that fluorinated silanes initially formed fluorine-rich surfaces, but that only a fraction of the molecules remained strongly bound after high-temperature annealing. This work demonstrates that immersion ellipsometry is a robust and sensitive method for characterizing ultrathin silane films and for resolving structure-property relationships that cannot be reliably accessed using conventional ellipsometric approaches.