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E. O'Connor¹, F. Steger¹, C. Sturzenegger¹, D. Schachtler¹, H. Thomé¹

Metal fluorides are of interest for various applications given their properties of low refractive index, wide bandgap and high transparency. Physical vapour deposition techniques have most frequently been used to deposit Aluminium Fluoride (AlF3). In recent years there have also been a few reports on the growth of AlF3 via ALD. This is of increasing interest for optical applications, particularly where conformal coatings are required on component geometries of increasing complexity. In this work AlF3 films were deposited using a PEALD process with a trimethylaluminum (Al(CH3)3) precursor and a sulfur hexafluoride (SF6) plasma. The depositions were performed in an Oxford Instruments FlexAl ALD system. A number of the PEALD parameters were optimized to deposit high quality AlF3 films. Spectroscopic ellipsometry results indicate that the material has a low refractive index of 1.35 (@633nm). Atomic Force Microscopy demonstrates that the layers have low surface roughness comparable to the underlying substrate. The film density of 2.95 g/cm3 as measured through X-ray reflection compares favourably to densities previously reported for both thermal and plasma enhanced ALD of AlF3. Compositional analysis to examine the incorporation of impurities shows no evidence of residual sulfur in the layers from the SF6 plasma process. The optical performance of the films was also studied with regard to the PEALD process parameters, and relevant results will be presented including Laser Induced Deflection, Laser Induced Damage Threshold and Total Integrated Scattering.

¹RhySearch, Forschung und Innovationzentrum Rheintal, Werdenbergstrasse 4, 9471 Buchs Switzerland