Enhancement of the annular illumination effect using a printing process resolvable with low image contrast

Resolution enhancement by annular illumination is discussed. The image contrast of patterns on a wafer is improved by annular illumination in high spatial frequency regions. However, it is still less than 0.5, although the ordinary resist patterns can be resolved only when the image contrast is over about 0.6. Therefore, the actual resolution limit is hardly extended at all by the ordinary resist process. To utilize the small improvement of the image contrast and to realize a noticeable resolution enhancement, a special resist process, by which fine patterns can be printed with a low image contrast of less than 0.5, should be adopted. A large resolution enhancement is actually demonstrated by combining an extra-thin resist process with annular illumination. The optimum annular illumination conditions for low-image contrast printing are also investigated, and it is clarified that a large annular source radius is preferable. Since the resist process used is a matter of choice, various surface imaging techniques will also be applicable.