Hard antireflection coatings with enhanced mechanical properties based on gradient structure

Antireflection (AR) coatings with sapphire-like hardness are highly desired in various applications. Traditionally, hard AR coatings resist penetration, scratching, and abrasion by depositing a hard layer, such as silicon nitride (Si3N4), with a 1 to 2 mu m thickness. However, thick Si3N4 thin films, fabricated by high-energy inductively coupled plasma assisted (ICP-assisted) sputtering, often introduce high residual stress, leading to severe buckling problems when the surface is scratched. To address these challenges, we employ a "Step up-step down" design method, integrating a hardness gradient structure with a series of SiOxNy films. This approach achieves high optical transmittance (Tave > 98.7 % at 420-720 nm), low residual stress (similar to 680 MPa), improved scratch resistance, and strong adhesion at the film-substrate interface (L-C3 similar to 180 mN). Our findings demonstrate significant potential for various mechanical and optical applications, including automotive and consumer electronics devices.