Evanescent Wave Assist Features (EWAFs) are features that are sensitive to near-field radiation that modify diffracted order intensities from photomask patterns. In implementations studied in this thesis, the EWAFs increase a transmitting feature's image contrast and Normalized Image Log Slope (NILS). In this way, the EWAFs are a way to improve image fidelity for high-resolution features. The assist features consist of local, buried grooves located around transmitting mask regions. These grooves reside in otherwise unused areas, since they are located under or on top of opaque mask absorber regions. In these "buried" locations, they are not optically visible to the lithographic system in a traditional sense. Designs are explored for both top-surface and bottom-surface EWAFs on 1-D and 2-D layouts. Using EWAFs, 27% image contrast improvements have been shown on contact layouts, as well as best-case image contrast improvements of over 2X on 1-D slot-type mask layouts. Dependence of EWAF effect on mask absorber material and bottom-surface relief shape is studied, as well as polarization sensitivity and the role of Surface Plasmon Polaritons (SPP). TM polarized light creates a normal-component field enhancement that amplifies surface waves across suitably conductive absorbers. These waves can then interact with bottom-surface EWAF grooves, and convert to propagating based on grating action. The converted orders may then interact with standard transmitted orders from a transmission feature, resulting in enhancement or suppression, depending on EWAF tone, pitch regime, and illumination angle. A demonstration EWAF sample, as well as a reference sample with no grooves, was fabricated at the RIT SMFL and tested using a Variable Angle Spectroscopic Ellipsometer (VASE). Accounting for pitch deviations during fabrication, as well as lateral inter-layer alignment offsets gives diffracted order responses that agree with SPP resonances observed in the samples at normal incidence and diffracted order enhancement factors that agree with simulation.
Library of Congress Subject Headings
Microlithography; Semiconductors--Etching; Imaging systems--Image quality
Department, Program, or Center
Microsystems Engineering (KGCOE)
Lafferty, Neal, "Evanescent wave assist features for optical projection lithography" (2011). Thesis. Rochester Institute of Technology. Accessed from
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