Resolution enhanced proximity printing by phase and amplitude modulating masks

Photolithography based on proximity printing offers a high throughput and c ost effective patterning technology for production of, for instance, large area liquid crystal displays. The resolution of this technique is limited d ue to wave-optical effects in the proximity gap between the binary amplitud e mask and the substrate. We can improve the resolution drastically by repl acing the conventional photomask with a mask causing both amplitude and pha se 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 mum gap, a 3 mum line/space pattern is resolved clearly for visibl e light illumination, whereas under conventional conditions the image is co mpletely degraded. The proximity mask used in our experiments was fabricate d by e-beam lithography with four height levels and two amplitude transmiss ion values.