Linear phase ring illumination monitor

A phase shifting mask pattern consisting of an Airy pattern multiplied by a linear phase progression is introduced as a quantitative monitor for the angular distribution of illumination in optical projection printing. The proximity effect spillover from the pattern rings is in phase at the center position only for illumination from the designed off-axis illumination direction. Thus, a quantitative analysis of the intensity in a particular pupil location is available simply by measuring intensity at the center of the pattern. A theoretical analysis is presented and image simulation studies are made of an implementation based on four-phase mask making. The results show that the signal strength is high (about 1/3-1/5 of the clear field intensity per ring), the angular discrimination is good (about 0.5/ring number), and that linear phase rings are reasonably unaffected by aberrations. Mask requirements suggest this technique is most advantageous for monitoring dipoles or quadrupoles. Since the patterns print at normal exposure levels, they may also be included on product wafers to fingerprint the illumination during production. (C) 2003 American Vacuum Society.