Study of the moisture and thermal resistance of AlN/ZrN/AlN multilayers coating

The moisture and thermal resistances of AlN/ZrN/AlN multilayers deposited by magnetron sputtering onto borosilicate glass wafers were investigated. This kind of multilayers has a great reflectivity in the IR range and a good transmission in the visible range. Two deposition parameters (substrate temperature and substrate bias) were varied. The as-deposited coatings were annealed in an oven at different temperatures for 10 min and/or exposed to a corrosive atmosphere (200 ml SO2 into 2 l of water heated at 40 °C) for three cycles of 8 h. The degraded films were characterised by different techniques. Spectrophotometry in the UV-Vis-IR range (300 to 2500 nm) was used to measure the optical properties (Transmission, Reflection). X-ray photoelectron spectroscopy (XPS) was used to measure the films stoichiometry and to characterize the humidity migration into the layers. The substrate RF bias has a great influence on the coating stability. The ZrN layer oxidation occurs during the moisture and thermal tests when the layers were deposited with a bias below 35 W and the optical properties of the coatings are degraded. Above this value, the ZrN layer is well protected from humidity diffusion. This behaviour is certainly due to the porosity decrease of the films when bias is added. These results confirm the necessity of a bias during deposition to increase the coating durability.