Anodized titanium oxide thickness estimation with ellipsometry, reflectance spectra extrema positions and electronic imaging: importance of the interfaces electromagnetic phase-shift

The oxide thickness of anodized titanium samples has been determined through ellipsometry, reflectance spectra extrema positions and electronic imaging. The reflectance spectra extrema position technique is applicable in the case were the oxide layer is thin enough to generate an interference phenomenon inside the oxide layer. When reflected at the air/oxide and oxide/metal interfaces, the electromagnetic field undergoes a phase-shift, which is often neglected in the literature. By comparing the oxide thickness obtained through the different techniques, it is shown that this phase-shift is not negligible for thin oxide layers. The relative error on the oxide thickness is for example of about 50% for a 17 nm thick oxide layer. By studying the discrepancy observed in the literature for the titanium and oxide layer refractive indexes, which is of about 13% in the wavelength range (350–600 nm), the error induced when neglecting the electromagnetic phase-shift is higher than the error induced by the uncertainty on the refractive indexes for oxide thicknesses below about 50 nm.