Sub-wavelength coatings on metals by ALD

Abstract

The equations that describe reflection and transmission of electromagnetic field on various interfaces of transparent materials can be found from almost any textbook on optics. A lot less known problems are related to the behavior of e.m. field on such interfaces where one part is a dielectric material and the other one metal layer. In such cases one can for example modify the colour of the metal in the reflected visible light by varying the film thickness. In most common cases the metal surface is being oxidized either spontaneously or by a controlled treatment. Then the visible light normally becomes transmitted through the oxide layer and may experience a number of internal reflections between the metal/oxide and the oxide/air boundaries leading to scattering and absorption losses and to a large phase change.

In this thesis the main focus has been to calculate and experimentally test the effect of an oxide layer on the reflected light by taking into account higher order reflection events in the analysis One motivation was to test the applicability of the atomic layer deposition method for modifying the optical response of metal surfaces by highly controllable sub-wavelength metal oxide coatings.

The atomic layer deposition (ALD) has become a prominent technique for conformal deposition of thin films. By using ALD one can get pinhole free films of almost perfectly uniform thickness in this work ALD was used to deposit optical oxide coatings with the main emphasis on very thin sub-wavelength thick films on metals.

One application of optical coatings that was tested in this work is an optical polarizer. The idea is to reduce one polarization component of light as effectively as possible. In order to find the behavior of the two polarization components of the light field in thin optical coatings the physical system was simulated using Matlab.

In the experimental part of the work, a polarizing component was fabricated by using ALD to make a 229 nm thick A1203 film it was also experimentally tested by measuring reflected light using the monochromatic (635 nm laser) source in a setup that included a goniometer Rather satisfactory agreement between the experimental reflectance curve and the simulated data was found.