Determining both the refractive index and thickness of few-nm films by ellipsometry is no longer impossible. Measuring in a fast and non-destructive way, the unknown refractive index n and thickness t of few-nm films is an important challenge for applications ranging from bio-sensing to optoelectronics. In view of its capability for the non-contact detection of very thin films, ellipsometry seems adequate to achieve this goal. However, the usual procedures for retrieving both n and t from ellipsometry measurements are known to fail if the film is too thin. Usually, a fitting of the measured ellipsometric angles is performed using Fresnel equations where n and t are the adjustable parameters. For t < 15 nm, a satisfactory agreement with the measured data within the experimental error is achieved with several markedly different and yet unphysical (n,t) sets. In their paper, Peter Nestler and Christiane Helm propose a method to overcome such long-known indetermination. From a second-order development of Drude transition layer equations, they show that a couple of ellipsometry measurements yield two uncorrelated moments, from which both n and t can be determined with precision. The reliability of the method is demonstrated for 5-nm-thick polymer films.

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