Characterization of amorphous carbon films from 5 nm to 200 nm on single-side polished a-plane sapphire substrates by spectroscopic ellipsometry

In this work, a series of amorphous carbon films were deposited on a-plane sapphire substrates by magnetron sputtering with deposition time from 15 min to 8 h, in order to investigate the thickness and optical properties in the process of growth in a non-destructive way. They were characterized by using Mueller matrix spectroscopic ellipsometry together with topography profilometry and Raman spectroscopy. Two models of a Bruggeman effective medium approximation model and a single Cody-Lorentz oscillator model have been proposed to fit films thickness and optical constants from Ultraviolet (UV) to visible (210 nm-800 nm), and Transmission Electron Microscope (TEM) has been used to verify the proposed model for thickness fitting results. The optical constants of the amorphous carbon film have been determined by fitting together all measurements in samples deposited for 2 h or more, with the film thickness being the only sample-independent parameter. The results show that the thickness from 5 nm to 200 nm can be characterized in a nondestructive way although there is a relatively large thickness error compared with the Transmission Electron Microscope results for thin films (d < 20 nm) when the deposition time is less than 2 h because of the nonuniform deposition in the beginning. The relative error between the TEM and Spectroscopic Ellipsometry results can be reduced to 1% after 4 h sample. That means spectroscopic ellipsometry can still provide an indicator for the trend of thickness growth.

Automated summary

In this study, the thickness and optical properties of amorphous carbon films deposited on a-plane sapphire substrates were investigated non-destructively using Mueller matrix spectroscopic ellipsometry. The films were characterized using topography profilometry and Raman spectroscopy. Two fitting models, Bruggeman effective medium approximation model and single Cody-Lorentz oscillator model, were proposed to determine the film thickness and optical constants. The results were verified using Transmission Electron Microscope (TEM). The findings showed that non-destructive measurement of film thickness was possible in the range of 5 nm to 200 nm, with some error for thin films due to nonuniform deposition. However, after 4 hours of deposition, the relative error between TEM and Spectroscopic Ellipsometry measurements reduced to 1%, indicating the potential of spectroscopic ellipsometry in monitoring thickness growth.