Optical characterization of a four-medium thin film structure by real time spectroscopic ellipsometry: amorphous carbon on tantalum.

A real time spectroscopic ellipsometry (RTSE) investigation is reported for comprehensive optical characterization of a thin film/substrate system, vacuum-deposited hydrogenated amorphous (diamondlike) carbon on tantalum. Precision RTSE measurements were carried out from 1.6 to 4.0 eV with a time resolution of 3 s and a repetition period of 15 s. Spectra collected during substrate exposure to an Ar(+) beam identify processing conditions for optimum precleaning and provide the bulk dielectric function of the Ta. Spectra collected during alpha-C:H growth to approximately 1700 A by ion beam deposition are best interpreted with a four-medium model (Ta/interface/alpha-C:H/ambient). From the analysis, we deduce the bulk dielectric function for alpha-C:H and an approximate dielectric function for the interface layer, interpreted as a carbide. The time evolution of the bulk and interface layer thicknesses is also determined. The dielectric functions of alpha-C:H deduced assuming a four-medium model and a three-medium model which neglects the interface layer differ by as much as 5%. The capabilities of measurement and analysis reported here represent a major new advance in the optical characterization of thin films.