Reliable modeling of ultrathin alternative plasmonic materials using spectroscopic ellipsometry [Invited]

Spectroscopic ellipsometry is a prominent method for finding both the thickness and permittivity of unknown thin material films due to its sensitivity, flexibility, and self-referencing nature. For non-absorbing films, the thickness and permittivity can be readily retrieved due to an excess of data content, which produces a clearly defined best-fit for a series of test material parameters. However, in materials with absorption throughout the spectrum, there is often insufficient data content to uniquely characterize both film thickness and permittivity for the thin film material. This leads to a flat fit optimization curve that can produce apparently good fit results from a wide range of material parameters. To overcome this data content shortage, additional techniques are necessary to either increase the measured data content or reduce the unknown parameters and establish the unique material properties for the film. Here, we explore the use of spectroscopic ellipsometry combined with transmission intensity data and discuss the pitfalls of fitting such thin absorbing films. Specifically, we examine the case of titanium nitride, a rising refractory alternative plasmonic material and demonstrate that without proper ellipsometry fitting procedures, retrieved permittivity values can vary by a factor of three or more.