Development and optimization of atomic layer deposition nanolaminate protective coatings

Abstract

Protective coatings for metallic mirrors must resist environmental and chemical degradation while preserving the reflectance of the underlying surface. Atomic layer deposition (ALD) provides the precise thickness control and conformality required for ultra-thin protective barriers. However, plasma-enhanced ALD (PEALD) exposes oxidation-sensitive metallic substrates to reactive plasma species, while single-layer coatings may remain limited by material-specific properties and growth defects. This work develops and optimizes PEALD oxide nanolaminate coatings deposited at 150 ˝C for the protection of silicon, silver, aluminium, and gold mirror substrates. An Ar/H2 plasma pretreatment (50W, 2% H2, 60 s) was selected as providing suitable surface preparation while causing minimal optical and morphological modification. Silver was identified as the most oxygen-sensitive substrate, with its reflectance at 532nm decreasing by more than 25% after only 30 s exposure to a 50W O2 plasma. A thermally deposited Al2O3 interlayer was therefore introduced to protect the substrate during subsequent PEALD processing. A thickness of 4nm suppressed the plasma-induced reflectance loss comparably to 8nm while limiting optical interference. Nanoscratch testing further showed consistently higher first critical loads for Al2O3 than for a PEALD AlN interlayer across all investigated substrates. Five 50 nm single-layer oxides, Al2O3, SiO2, TiO2, HfO2, and Ta2O5, were screened under HCl and KOH exposure using reflectance measurements and visual inspection. Based on this screening, SiO2, TiO2, and HfO2 were selected for nanolaminate development. TiO2/SiO2 and HfO2/SiO2 architectures with individual layer ratios of 1:1, 5:5, 4:1, and 1:4nm were subsequently evaluated in 10 wt% HCl for exposure times of up to 90 min. TiO2/SiO2 nanolaminates showed superior overall performance compared with HfO2/SiO2, while SiO2-rich architectures provided the highest chemical stability. In particular, the 1nm TiO2:4nm SiO2 architecture provided the best balance between reflectance preservation, resistance to delamination, and chemical protection. Comparison of total coating thicknesses of 25, 50, and 100nm identified 50nm as the most suitable compromise between protective performance and preservation of optical properties. Potentiodynamic polarization measurements in 0.1M HCl and electrochemical impedance spectroscopy in 0.1M citric acid at pH 5.2 further supported the favourable barrier performance of the selected architecture, which exhibited low current densities and impedance magnitudes on the order of 107 Ω–108 Ω. Overall, the results demonstrate that combining complementary oxides within a controlled nanolaminate architecture can improve the chemical protection of metallic mirrors while maintaining their optical functionality.