Evaporation and ion assisted deposition of HfO2 coatings: Some key points for high power laser applications

Abstract

Optical interference filters made by layers of optical coatings are used to shape and transport laser beams. If a defect is present in the stack (cosmetic defect, stoichiometry defect, absorption band…), and high laser power density is reached, a strong energy into the material can be deposited involving the destruction of the coatings (either by melting or mechanical failure). One of the key methods to achieve high performance coatings is to reduce such defects as much as possible. Hafnia (HfO2) coatings are undoubtedly one of the most successful materials for high power laser applications. Associated with a low index material such as silica, high laser induced damage threshold (LIDT) interference filters can be achieved. Hafnia is often the LIDT limiting factor. During the evaporation HfO2 particles create buried defects in the deposited material. These defects are sources of the coating damage during laser irradiation. To avoid this phenomenon we have evaporated metallic hafnium that was oxidized by oxygen inlet into the vacuum chamber or use an oxygen ion beam bombardment during the film growth. Argon or, better, xenon ions in the beam produced densities as high as 99% of the bulk, with low water content, and an improved optical transmission in the mid infrared window. This article deals with the optimization of such a deposition process regarding coating density, mechanical stresses, and optical properties (optical absorption). Films deposited by HfO2 evaporation are compared especially using photothermal mapping. The stoichiometry of the defects determined thanks to localized Auger spectroscopy clearly indicates how injurious such defects could be for high laser flux applications. Finally, laser damage thresholds of the films at 1.06 μm are measured and possible damage mechanism discussed.

Optical characterization of hafnium oxide thin films for heat mirrors

“…HfO 2 is preferred for optical devices such as optical filters, high-reflectivity mirrors, high power lasers and other optoelectronic devices because of its desirable properties such as high refractive index, high laser damage threshold and high transparency in the visible and ultraviolet (UV) spectral ranges [1][2][3][4][5]. Because of its outstanding chemical stability, electrical and mechanical properties, high-dielectric constant and wide band gap, hafnium dioxide (HfO 2 ) has been considered as one of the most important materials with a wide range of potential scientific and technological applications in electronics [6,7], magneto-electronics [8,9], optoelectronics [10] and metal oxide semiconductor devices [11].…”

Effect of RF power of post-deposition oxygen treatment on HfO 2 gate dielectrics

“…The higher binding energy in Hf 4f peak for the HfO 2 film represents the Odeficient feature. A decreasing binding energy in the Hf 4f peak has been reported for the case that the resulting metal oxide film is fully oxidized which approaches to stoichiometric state [17]. Therefore, the oxygen vacancies can be repaired by post-O 2 plasma treatment.…”

Laser-induced damage measurements on phase-unifying mirrors for XeF excimer laser cavities

“…It is worth mentioning that the technology of evaporation of HfO 2 has been optimized for high LIDTs values. 9 The thickness-profiled SiO 2 layer was realized by inserting a proper mask with a central hole 3 mm-in-diameter in contact with the substrate.…”