Non-quarter-wave dielectric mirror prepared by thermal atomic layer deposition

Abstract

In this work we design and fabricate from n(λ) and k(λ) experimental data for both Al2O3 and TiO2 single layer materials, an optical coating as “dielectric-mirror” following the non-quarter-wave stack formula (HxLy)8Hx. Optical coating based on multilayer film on BK7 glass and Si(1 0 0) wafer substrates, was grown by thermal atomic layer deposition at 150 °C. Optical constants and optical properties of the TiO2 - Al2O3 multilayer stack, before and after thermal treatment at 450 °C, were studied via spectroscopy ellipsometry and UV – Vis measurements in the spectral range from 200 to 1100 nm. Also, similar samples were studied by means of TEM, SEM and AFM at room temperature in order to obtain information about the morphological properties. From optical studies, we found absorption due to carbon impurities related with organometallic precursor used in ALD process; to reduce carbon related absorption, samples were submitted to an annealing process at 450 °C under air atmosphere. A reject zone or “stopband region” between 381 and 451 nm, with maximum reflection around of 99.9%, cut-off points at 371 and 455 nm after thermal treatment. This reject zone presents an acceptable bandwidth at λ0 = 420 nm reference wavelength. Results open the possibility to fabricate dielectric-mirrors on complex geometry substrates without the restriction of direct evaporate exposed-view, due to the conformality advantage of ALD technology and its affinity with nanophotonics and integrated optics.