Chapter 4 introduced important methods to enhance the resolution limits of lithography by modifying components of the optical imaging system, especially the mask and the illumination. These optical resolution enhancements aim to improve the image or intensity distribution that exposes the photoresist. This chapter presents important innovations that improve the pattern transfer of smaller features from given, diffraction-limited images into a spatial modulation of the photoresist or of other materials. The described techniques exploit specific (nonlinear) material properties and combinations of different materials and processing techniques. Therefore, we call them material-driven resolution enhancements.

This chapter starts with a brief review of some optical aspects of the resolution limit. A general patterning strategy is devised that enables the manufacturing of smaller patterns by appropriate superposition of multiple images and/or multiple process steps. The following sections introduce several specific double-exposure and double-patterning techniques that are used to push 193 nm immersion lithography to feature sizes below 45 nm. Directed self-assembly offers interesting options for a more-cost-effective scaling. The last section provides a brief overview of thin-film-imaging techniques that split the functionality of the photoresist into several materials and processes.