High-resolution proximity printing by wave-optically designed masks

Proximity printing is a high throughput and cost effective lithographic technique for production of e.g. large area flat panel displays. The resolution of this technique, however, is limited due to diffraction effects that occur at mask pattern edges. We can improve the resolution drastically by replacing the conventional photomask with a mask which compensates these diffraction effects. The resulting mask modulates phase and amplitude of the exposure beam in such a way that the required image is formed at a predetermined distance behind the mask. In this work we describe the basic principles behind the resolution enhancement technique and explain the mathematical model to design this new type of proximity mask. The feasibility of the method is demonstrated by theoretical calculations as well as by practical experiments. The diffractive structures that have been examined are designed to form an image at a distance of 50 micron behind the mask. The mask contains 2 amplitude and 4 phase levels and the pixel size is 1 micron. Under these conditions a 3-micron line/space pattern is clearly resolved, whereas under conventional conditions the image is completely distorted.