From coffee rings to slopes: solid content effects in inkjet coating for nanoimprint lithography

Abstract Emerging photonic technologies demand increasingly precise and scalable fabrication methods to meet the complex design requirements of next-generation optical devices. Uniform resin deposition is critical for applications such as augmented reality and virtual reality (AR/VR) waveguides. Nanoimprint lithography (NIL) is a high-resolution, cost-effective patterning technique that allows for the production of such intricate 3D structures in a single process, accommodating a broad spectrum of sizes and shapes ranging from nanometer dimensions to several hundred micrometers. Precise control of resin layer thickness and uniformity is critical for optimizing the functionality of the final photonic device. Inkjet printing complements NIL by offering flexible, material-efficient deposition of customized patterns and thicknesses, thereby supporting the fabrication of next-generation photonic devices with reduced waste compared to conventional methods like spin coating. AR/VR waveguides require multiple form factors within a single device, necessitating tailored layer thicknesses to maintain continuous residual layer thickness across varying geometries. While inkjet materials typically perform well within a defined thickness range, this range is limited. Thin layers may result in incomplete droplet merging due to insufficient reflow, leading to surface roughness or non-closed coatings. Thicker layers, on the other hand, intensify reflow dynamics and the coffee ring effect, producing non-uniform edge features. When printing thickness steps, this can result in sloped layers rather than the intended staircase profiles. This study investigates how varying the solid content of UV-curable NIL resists affects edge quality and overall layer formation. By focusing on minimizing the coffee ring effect and achieving homogeneous coatings, the work highlights the importance of tuning ink formulation in conjunction with printhead capabilities and substrate properties. The results demonstrate that careful adjustment of solid content can significantly improve layer uniformity and edge definition, contributing to more reliable and precise inkjet-based NIL processes.