Development of an innovative low-temperature PEALD process for stress-compensated TiO2 and SiO2 multilayer anti-reflective coatings

This study presents a low-temperature plasma-enhanced atomic layer deposition (PEALD) technique for fabricating high-performance, stress-reduced anti-reflective coatings (ARCs). To the best of our knowledge, this is the first extensive study on titanium dioxide (TiO₂)/silicon dioxide (SiO₂) stacking with PEALD at such a low temperature of 70 °C, which may help to overcome high-temperature deposition issues and mechanical stress for polymer substrates. Despite the presence of impurities in the low-temperature deposited films, the measured extinction coefficient (k < 10^-4) indicates negligible optical absorption in both TiO₂ and SiO₂ layers, ensuring optimal performance for ARCs. Stress compensation is observed between tensile TiO₂ films (≈ 220 MPa) and compressive SiO₂ films (≈ - 35 MPa). For multi-layer ARCs, this combination strategy leads to a very low total stress of 48 MPa, which is a big step forward for stress control in optical coatings. This stress-reduction effect remains effective even when the thickness difference reaches up to 9.6%. This consistency has been demonstrated in real-world applications, where achieving an ideal level of thinness can be challenging. The optimized process at 150 W plasma power produces high-quality optics with an average reflectivity of 0.35% in the visible range while maintaining low stress, a significant achievement in low temperature deposited optical coatings. The choice of common, cost-effective materials like SiO₂ and TiO₂ makes this approach easily scalable for industrial use and can see the future of manufacturing ARCs for various applications. These films are characterized by a low density of defects and an amorphous structure with the smoothness of their surface being close to one atomic monolayer (≈ 0.2 nm), which indicates their high optical quality, comparable to films deposited at high temperatures. The low-temperature PEALD presented in this work not only pushes the boundary in advanced optical coatings but also enlarges the capacity in coating temperature-sensitive substrates and complex 3D structures. This innovation paves the way for applications in bendable electronics, high-performance optical components, and next generation display devices.