Determining thickness and refractive index from free-standing ultra-thin polymer films with spectroscopic ellipsometry

摘要

Highlights • Thickness and refractive index are uniquely determined from spectroscopic ellipsometry characterization of free-standing polymer films with thickness down to 5nm. • There is low thickness-index correlation for case of free-standing, ultra-thin films. • Ultra-thin free-standing films show orthogonal SE data separation: Ψ is affected by index while Δ is affected by thickness. Abstract It is a well-known challenge to determine refractive index (n) from ultra-thin films where the thickness is less than about 10nm [1,2]. We discovered an interesting exception to this issue while characterizing spectroscopic ellipsometry (SE) data from isotropic, free-standing polymer films. Ellipsometry analysis shows that both thickness and refractive index can be independently determined for free-standing films as thin as 5nm. Simulations further confirm an orthogonal separation between thickness and index effects on the experimental SE data. Effects of angle of incidence and wavelength on the data and sensitivity are discussed. While others have demonstrated methods to determine refractive index from ultra-thin films [3,4], our analysis provides the first results to demonstrate high-sensitivity to the refractive index from ultra-thin layers.