A new perspective on proximity printing: From ultraviolet to x ray

Abstract

Proximity printing is characterized in terms of linewidth variations in the gap–exposure (E–G) space. The usable E–G regions of all feature shapes and sizes on a given mask design have to be superimposed and the common region defines the depth of focus as well as the working distance for a given exposure latitude and linewidth tolerance. In this paper, the E–G diagrams are constructed and studied for 0.25, 0.35, and 0.5 μm x-ray features at mask-to-wafer gaps between 10 and 80 μm, or equivalently, 2.5, 3.5, and 5 μm features for 250-nm-deep-UV light at mask-to-wafer gaps between 4 and 20 μm. The types of illumination include collimated light, finite source size, and ghost line suppression. Linewidth tolerances of ±25 and ±16 nm for perfect and imperfect masks are examined. The effects of 50 nm positive and negative print biases are studied. The depth of focus and working distance are given for five representative lithographic features consisting of equal lines and spaces, isolated line openings, isolated opaque spaces, holes, and islands as well as for only the three long features.