Measurement of Spherical Aberration Utilizing an Alternating Phase Shift Mask

Abstract

A spherical aberration function is measured utilizing an alternating phase shift mask (PSM). First, it is theoretically considered that with a highly coherent illumination, the focus of the alternating phase shift lines and spaces (L/S) pattern is equivalent to that of rays in a small annular area of the reference sphere when the aberration function is axially symmetric in the sphere. Then, the focus variation with the pattern pitch is obtained by a printing experiment that employs an alternating PSM. After a primitive mathematical operation, the aberration function is obtained from the experimentally observed focus variation. The aberration function is examined by investigating various anomalous phenomena observed in the printing with the various resolution enhancement techniques (RETs). The comparison between the experimentally observed phenomena and the predictions based on the calculated images with the measured aberration functions shows fair agreement. Consequently, the validity of this measurement method is confirmed.