Precise determination of the refractive index of sputtered MgO thin films in the visible light range

A technique to determine optical constants of thin transparent films is discussed. Instead of the combination of the transmittance and the reflectance at a particular wavelength, the transmittance spectrum for a wider wavelength range is analyzed. The analysis is based on a fact that the dispersion of the complex refractive index n - ik of the film body can be described well with four parameters in the Lorentz theory. The surface roughness and the film thickness are also taken as fitting parameters, and hence the optical behavior of the film can be calculated with these six parameters. A set of best parameters is determined so that the square sum of the difference between the calculated transmittance and the observed one over the visible spectral range can be minimized. MgO films were prepared on glass substrates by the rf magnetron sputtering from a MgO target. The sputtering was carried out in the mixture gas of Ar and O-2 at 1 Pa. Fairly transparent films of various thickness (< 0.25 <mu>m) could be prepared. The transmittance spectrum was analyzed and the six parameters were determined. The parameters were found to be almost the same in all samples except the thickness and the roughness. The refractive index was 1.72 +/- 0.01 and the extinction coefficient was less than 3 x 10(-4) at the wavelength of 633 nm. The parameter of the surface roughness agreed well with the roughness value which was measured using the atomic force microscope. (C) 2000 Elsevier Science Ltd. All rights reserved.