M-Lines Spectroscopy for Thin Films: A New Perspective

The m-lines spectroscopy is a precise, non-destructive and contactless method, and one of its main applications is the determination of the geometric-optical parameters of a thin film deposited over a substrate, namely the refractive index and the thickness of the film under analysis. The method has been first described in the seminal work of Tien et al. [1] more than half a century ago and, since then, it has been reported in the literature that at least two modal indexes of the same polarization are required to unequivocally determine a given film’s refractive index and thickness. This constraint imposes a limit in the waveguide’s width, for it leaves out the possibility of determining the geometric-optical parameters of all films where only single mode propagation is feasible. In this work, we developed a strategy that, not only extends the width range calculation of the method down to single mode operation, but also the results’ analysis led us to conclude that by solely using the fundamental modes in the calculation, the method’s precision shall increase and the determined parameters should be closer to the physical reality of the film.