Resolution enhancement techniques for optical lithography and optical imaging theory

Production of a fine pattern is necessary to get a high integration degree of integrated circuits. The conventional methods which utilize high numerical aperture and short wavelength exposure are limited by designing and manufacturing of a practical lens and make the focus depth narrow. Resolution enhancement techniques (RETs) have, therefore, been required and proposed. This paper introduces a phase-shifting mask, a typical RET, points out the problems and inconsistencies of conventional optical imaging theory and explains the image formation concept of expansion of plane waves. Essentially using this concept, an attempt is also made to describe some other typical RETs with potential.