Recent progress in fabrication methods for micro structures on optical thin film

Abstract

Optical thin film with micro structures on the surface is expected to be widely applied on large-area optical devices such as thin film solar cells and flexible display, due to the enchancement of the light extraction efficiency or light utilization. Geometric characteristics and physical properties of micro structures such as shape precision, structural defects and residual stress, have comprehensive influence on the quality of optical thin film. Ultraviolet (UV) curing and hot embossing are two major processes for the fabrication of micro structures on the optical thin film. This review surveys the recent progress of UV curing and micro hot embossing in terms of polymeric material behavior, filling mechanism and demolding process. For the UV curing process, filling process is firstly discussed and the bubble defects, residual layer and size effect are systematicall analyzed. Then, the curing mechanism is well summarized and the effect of UV dose on the physical properties of micro structure is futher explored. Finally, the challenges in the demolding process for micro structures with high aspect ratio are cataloged and some corresponding approaches are proposed. For hot embossing process, thermoplastic polymer flow behavior is investigated and single-peak and dual-peak deformation molds are discovered. Novel methods, such as ultrasonic assisted and gas assisted process are addoped to improve the filling height. Besides, demolding temperature and demolding force are two major factors for the micro structure defects. In addition, mold fabrication techniques are comprehensively summarized into direct machining method and indrect machining method. Some innovations are also introduced for the roller mold fabrication. Furthermore, the recent advancement of roll-to-roll apparatuses for the UV curing process and hot embossing process is systematically cataloged as well. The highest rolling speed reaches 5 and 3 m/min for R2R UV curing process and R2R hot embossing process, respectively. However, the current status is still far from the final destination of 60 m/min for the large-scale industrial application. Therefore, in order to meet the final destination, technical challenges and future trends are presented for fabrication methods of micro structrues on optical thin film, in terms of size effect modeling in micro or submicro scale, process control for the comprehensive performance of micro structures, large-area mold fabrication technologies with high-precision and high-consistency, and coordinate control technology for the multi rollers in roll-to-roll movement.