In situ optical characterization during MPACVD diamond film growth on silicon substrates using a bichromatic infrared pyrometer under oblique incidence

In this paper, we carry out in situ optical characterization of diamond films on silicon substrates using an infrared bichromatic pyrometer under oblique incidence. We develop an optical model taking into account the effective emissivity of the diamond film/silicon substrate system. This model allows the determination of the diamond and silicon optical constants, the system real temperature, the incubation period and the growth rate, from the apparent temperature measured during the diamond growth. These parameters are found from the experimental data and from minimizing the mean square error using the Levenberg-Marquardt algorithm. The model can be extended to other partially transparent deposits on absorbent substrates at any observation angle and the first-hand values of the film optical constants can be found without the knowledge of the substrate optical properties. The pyrometric results thus obtained on the diamond films deposited by microwave plasma-assisted chemical vapour deposition in a CH4-H-2 gas mixture are discussed and compared to those deduced from Raman spectroscopy in order to investigate the diamond purity. As the CH4 concentration is augmented, the decrease in the diamond purity thus found is in agreement with the increase in the film extinction coefficient evaluated from pyrometry. Moreover, the behaviour of the apparent temperature pseudo-period provides information on the growth process with respect to the growth rate and the film real index variations.