Contrast mechanisms in high-resolution contact lithography: A comparative study

We compare three different approaches to high-resolution contact lithograph y with special emphasis on contrast mechanisms for subwavelength structures . Masks with protruding metal absorbers, masks with absorbers embedded in t he transparent background, and masks with air gaps and recessed absorbers a re studied. Using the Green's tensor technique we compute the light intensi ty distribution in the photoresist. The intensity and contrast functions ar e investigated for different mask geometries (absorber thickness, height of protruding elements), and the difference between chrome and gold as absorb er material is discussed. Our results show that embedding the absorbers in a transparent mask material enhances the transmitted intensity and the cont rast compared with a mask having protruding metal absorbers. A further impr ovement is achieved by a topographically patterned mask with air gaps and r ecessed absorbers. Optimized mask dimensions can be found for which the con trast and the depth of focus are increased. (C) 2001 Elsevier Science B.V. All rights reserved.