FUNDAMENTAL PRINCIPLES OF PHASE-SHIFTING MASKS BY FOURIER OPTICS - THEORY AND EXPERIMENTAL-VERIFICATION

In this article, the imaging principles of projection steppers for opt ical lithography, using standard transmission masks, are reviewed, and compared with the image formation using various types of phase shifti ng masks (PSM), in order to get a better insight in the working princi ples of the various PSM techniques. The printability of periodic lines using a standard transmission mask is compared with the use of an alt ernating shifter PSM and the dependence of imaging on spatial coherenc e is discussed extensively. Furthermore the imaging of isolated lines is discussed, using a bright field standard transmission mask, a chrom eless phase edge PSM, and a dark field rim PSM. The coherence dependen ce of an alternated shifter is verified experimentally. Experiments al so confirm the superior performance of a chromeless phase edge PSM to print isolated lines.