Optical, composition, durability, and environmental properties of the microwave plasma-assisted sputter-deposited silicon nitride

A study is presented of the optical, mechanical, composition, durability, environmental, and structural properties of silicon nitride thin films deposited at room temperature using a vertical axis rotating drum-based microwave plasma-assisted sputter system. The deposited material composition was investigated using both X-ray photoelectron spectroscopy (XPS) and elastic recoil detection analysis with time of flight (ERDA-TOF). The results indicate that the layers present a uniform composition with thickness and despite the incorporation of impurities, the Si/N ratio remains close to stoichiometric <i>S</i><i>i</i>₃<i>N</i>₄. The deposited material provides homogeneous low dispersion-with a refractive index of approximately 1.98 over a wide transmittance range (350 nm to 5 µm). Additionally, the single layer silicon nitride was found to be durable with a hardness, Young's modulus, and compressive stress of 31 GPa, 240 GPa, and -454<i>M</i><i>P</i><i>a</i>, respectively. The coating was subjected to and passed external surface environmental tests covered by MIL-C-48497A durability requirements (adhesion, abrasion, and temperature/humidity cycling) and MIL-C-675C (para 4.5.9) salt spray. Moreover, 10,000 wipes on the TS1888 (para 5.4.3) sand slurry abrasion test were achieved with no visible signs of damage. A silicon nitride/silica multilayer durable broadband visible anti-reflection coating is demonstrated, passing the same range of durability and environmental tests as single layer silicon nitride.