Resolution enhanced proximity printing by phase and amplitude modulating masks

Abstract

Photolithography based on proximity printing offers a high throughput and cost effective patterning technology for production of, for instance, large area liquid crystal displays. The resolution of this technique is limited due to wave-optical effects in the proximity gap between the binary amplitude mask and the substrate. We can improve the resolution drastically by replacing the conventional photomask with a mask causing both amplitude and phase modulation of the illumination wave. We describe a wave-optical design procedure for such masks. The feasibility of the method is demonstrated by results from computer simulations and practical experiments. We show that, for a 50 µm gap, a 3 µm line/space pattern is resolved clearly for visible light illumination, whereas under conventional conditions the image is completely degraded. The proximity mask used in our experiments was fabricated by e-beam lithography with four height levels and two amplitude transmission values.