Experimental Study On The Optical Constants Of Typical Engineering Materials At High Temperature Using Ellipsometry

Optical constants are the most fundamental thermal radiative properties of solid materials. It has been found that optical constants are significantly influenced by temperature. The optical constants at different Temperatures are important for accurate radiative heat transfer calculation, and experimental verification of quantum simulation methods of dielectric functions. Yet currently, optical constants are still rather lacking at high temperatures due to the oxidation and thermal emission problems, which has become the restrictive issue of thermal radiation research. During the last decades, spectroscopic ellipsometry has been greatly developed, and become the most accurate method to determine optical constants. However, the reflection-based optical constant determination methods including ellipsometry are based on the Fresnel’s equation for perfectly smooth surface, which cannot be achieved in reality. Hence the accuracy of the determined optical constants will be influenced by the remained surface roughness. In this work, the effects of surface roughness on ellipsometric measurements, and the correction method are examined. The optical constants of eight typical engineering materials at different temperatures are measured by spectroscopic ellipsometry combining with accurate temperature-controlling and anti-oxide devices. The temperature dependence of the optical constants are investigated by classical dielectric models.Considering the incident angle during ellipsometric measurements, the reflective characteristics of rough surfaces under large incident angles and the deviation of specular reflectivity of micro rough surfaces from the Fresnel’s equation are experimentally and numerically investigated. The effects of roughness, incident angle and wavelength are examined. The results show that, the specular reflection characteristic of rough surface is dramatically enhanced with the increase of incident angle and the decrease of relative roughness.The accuracy of optical constants measured by reflection-based method can be improved by increasing incident angle and highly polishing the sample surface. If the Fresnel’s equation is employed to approximately calculate the specular reflectivity of micro-rough surfraces, the relative error of TM reflectivity dramatically rises when the incident angle approaches to the pseudo Brewster’s angle, while the relative error of TE reflectivity is much smaller, and decreases monotonicly with the decrease of incident angle.The effective roughness layer (ERL) model is a routine method to describe the differences of real surfaces from perfectly smooth surfaces, and employed for optical constant correction and surface roughness determination in ellipsometric experiments. In this paper, the effects of incident angle and roughness in these applications of the ERL model.The results show that, the ERL model efficiently decreases the relative error of the measured optical constants when it is applied for optical constant correction. The relative error of the ellipsometrically determined optical constants can be reduced below 1% after ERL correction for a rough surface with the roughness of 5nm. The correction effects can be futher improved by increasing the incident angle. When the ERL model is employed in the roughness determination, the relation between ellipsometrically determined ERL thickness and absolute roughness cannot be determined uniquely. The ratio between them decreases with the increases of relative roughness below 0.05. The optimum incident angle for roughness determination by ellipsometry increases with the relative toughness.In this work, the optical constants of four dielectric materials including Al2O3 (8-30μm), AlN (10-20μm), Si3N4 (5-30μm) ceramics and MgF2 crystal (15-30μm) are measured by ellipsometry within the temperature range of 300-773K. The Lorentz model parameters at different temperatures are obtained by fitting to the measured dielectric functions. The results show that, as the temperature increases, the absorption peak of the measured dielectric materials decreases, the peak position shifts to longer wavelength, and the attenuation coefficient decreases.Besides, the optical constants of four metallic materials including TC4 titanium alloy (1.7-17μm), Ni (1.7-17μm), Al (1.7-9μm) and Ag (1.7-12μm) are measured by ellipsometry within the temperature range of 300-773K. The results show that both of the refractie index and extinction coefficient of TC4 decrease with increasing temperature.The refractive index of pure metal increases while the extinction coefficient decreases with the increase of temperature.The effects of temperature on the optical constants increase with the wavelength. The parameters of Lorentz-Drude model and Drude model for TC4 titannium, Ni, and Al, Ag are obtained by fitting to the measured dielectric functions, respectively. It is found that the plasma frequency and damping coefficient decrease for TC4 while increase for Ni and Al with the increase of temperature. The plasma frequency increases while the damping coefficient decreases for Ag with the increase of temperature.