Application of Fresnel equations to improve interface reflection of inclined exposure and develop micro-mirrors for blu-ray DVDs

Abstract

The purpose of this paper is to explore the application of inclined exposure technology to fabricate thick film polymer optical microstructures on glass substrates. And to resolve the partial reflection of optical energy at interfaces causes the structural problem of unnecessary photo cross-linking, which can affect structural shape. This paper utilities inclined exposure technology to cause UV illumination to impinge on the substrate at a certain non-perpendicular angle and fabricate a three-dimensional optical-grade micro-mirror as part of an effort to develop an integrated, miniature blu-ray optical pickup-head. Although a transparent glass substrate was employed in the experiment, the system included mask glass (n m =1.53), glass substrate (n g =1.5), and photopolymerized polymer (n p =1.67). UV light tends to undergo partial reflection at the interfaces due to the materials' different refractive indexes. This optical phenomenon is particularly severe at higher exposure energies (∼6,000 mJ / cm2 ) in the fabrication of thick films. The Fresnel equations are used to calculate that the maximum interface reflection of optical energy is approximately 1.5 %. This paper therefore selects a medium with a matching refractive index (n gl =1.473) and removable anti-reflection technology in order to resolve the interface reflection problem, and applies this solution to 45-degree micro-mirror manufacturing technology, enabling a surface roughness of approximately 20 nm (λ/20, λ=405nm) as measured by WYKO.