Graded Spin-on Organic Bottom Antireflective Coating for High NA Immersion Lithography

Abstract

A spin-on organic Graded Bottom AntiReflective Coating (GBARC) was developed to enable appropriate reflectivity control during advanced photoresist patterning using 193nm immersion lithography up to 1.35 NA (limits of hyper-NA lithography using water as immersion fluid). This novel technology constitutes a simplified reflectivity control approach over more conventional schemes such as dual-layer BARCs, trilayer stacks or single layer BARCs. The GBARC layer is formed upon vertical phase separation of optically tuned polymeric components present in a GBARC casting solution during the spin-coating and post-applied baking steps. The substrate surface energy is found to dictate the direction of the spatial assembly for any given phase-separating polymer blend combination. The index of refraction (n193) of the GBARC polymeric components is approximately matched to that of the photoresist and substrate layers for optimum reflectivity control, while the extinction coefficient (k193) increases gradually towards the substrate. Analytical characterization of the varying optical constants throughout the GBARC film is achieved by Variable Angle Spectroscopic Ellipsometry (VASE) which allows for a quantitative description of the optical gradient present within the GBARC layer. The optical gradient sharpness is shown to be impervious to total GBARC thickness variations at constant relative polymer mass loading as well as changes in the relative mass loading of polymeric components at fixed total GBARC thickness. Also, the link between image profile afforded by various levels of reflectivity control and the mechanical stability of photoresist nanostructures is investigated at 1.2NA by designing GBARC formulations with variable residual reflectivity. Substrate reflectivity in excess of 1% is shown to limit the focus-exposure window due to pattern collapse. Last, adhesion of 193 nm photoresist patterns to a GBARC film is compared to a commercially available single layer BARC. Enhanced adhesion is demonstrated in the case of the GBARC surface, despite the low surface energy that typifies GBARC layers