Optical multilayer interference coatings rely on the refractive index differences and specific thicknesses of the low and high refractive index materials used in optical multilayer structures. An accurate control of important parameters such as film thicknesses, uniformities, and refractive indexes is demanding. Atomic layer deposition (ALD) inherently possesses many characteristics beneficial for obtaining fully conformal and uniform films of specific thicknesses with excellent repeatability. Additionally, the layer-by-layer deposition of the films allows tuning of the film stack properties, such as refractive index, which is an advantage when designing optical filters. By now, Al2O3 has been most often used as a low refractive index material in ALD made interference filters because of a lack of suitable SiO2 ALD processes. To lower the refractive index from that of Al2O3, we have developed and examined various ALD processes of aluminum silicate thin films. We concentrate on reporting the refractive indexes, growth rates, and compositions of the films as these parameters are vital for screening suitable ALD processes for optical applications. By varying the amount of silicon in the AlXSiYOZ thin films, the refractive indexes between 1.47 and 1.59 were obtained in this study. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3519497] All rights reserved.