Optical requirements for projection lithography

Abstract

An image modulation criterion is proposed as a simple optical performance parameter for lithography using positive photoresist. The modulation, or contrast, is defined in terms of the maximum and minimum intensity in the image of a line/space pattern as ( I MAX − I MIN)/( I MAX + I MIN). For lithography directly on integrated circuit wafers, a contrast in the range of 80–90% gives acceptable linewidth control. Universal curves are derived showing image contrast for diffraction-limited optical systems with partially coherent illumination and including defocus error. The allowable defocus is derived as a function of the contrast requirement and pattern linewidth. Based on these optical considerations, and the properties of positive photoresists, the useful resolution of state-of-the-art projection systems are shown to be in the neighborhood of 1.25 μm, and progress to a useful resolution of about 0.5 μm is forseeable.