Anti-reflection coating characterization using scattered polarized light

Samenvatting

An increase of the energy conversion efficiency of solar cells can be achieved by minimizing reflection losses. A thin film coating serves this purpose when using optimum values of its refractive index and thickness. Also, the presence of surface roughness decreases the reflection of sunlight. Consequently, advanced cell concepts combine thin films with surface texturing. Research and development of these concepts require characterization methods. Ellipsometry is a fast, non-destructive, optical measurement diagnostic which enables characterization of substrates and films with high precision. The use of ellipsometry as characterization tool requires an optical model to extract physical information from the measured data. The optical models used for thin film metrology are generally based on flat surface samples. This work explores the possibilities to extend the range of applicability of ellipsometry to rough surface samples. Polished, acid etched and alkali etched silicon samples were deposited with thin silicon nitride films using a DEPx plasma enhanced chemical vapor deposition system which makes use of the expanding thermal plasma technique. The samples were studied by means of reflectometry, scanning electron microscopy and atomic force microscopy. A single wavelength ellipsometer (?=632.8 nm) was used to measure all samples resulting in ellipsometric layer thickness trajectories for each type of surface roughness. Comparison of the layer thickness trajectories indicated the influence of surface roughness on ellipsometry. Experiments and simulations showed that this influence can be attributed to light scattering and depolarization effects. These effects can be reduced by increasing the angle of incidence during the ellipsometry measurements.The influence of the acid etched surface roughness on the ellipsometry output showed similarities with the influence of an angle of incidence offset. A physical explanation of this behavior is given based on a feature size of the surface roughness in the so called short wavelength regime. This behavior can be exploited for relative layer characterizing measurements on deposited acid etched samples in which a film free sample is measured for reference. The accuracy of these results is acceptable for layer thickness determination, but unacceptable for the determination of the refractive index of the layer. The influence of the surface roughness of the alkali etched wafers on ellipsometry can be explained using the effective gradient medium approximation in addition to the 'angle of incidence offset' model. This extension of the effective medium approximation is designed for the long wavelength regime. It is shown that ellipsometry can be applied to rough surface samples. The influence of the surface roughness on ellipsometry can be explained physically. Additionally, ellipsometry can be used to characterize thin films on top of rough surface samples within the short wavelength regime.

Datum prijs	31 aug. 2014
Originele taal	Engels
Begeleider	W.M.M. (Erwin) Kessels (Afstudeerdocent 1) & B. van Gerwen (Afstudeerdocent 2)