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

It is a well-known challenge to determine refractive index (n) from ultra-thin films where the thickness is less than about 10 nm [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 5 nm. 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.