Impact of spherical aberrations on printing characteristics of irregularlyaligned patterns of alternating phase shift mask

The printing characteristics of irregularly aligned patterns with alternati ng phase shift masks (PSMs) and the impact of spherical aberration on these characteristics are investigated by experiments and calculations of optica l images, In experiments, very poor printing characteristics are observed f or particular irregularly aligned patterns, even if there is no phase contr adiction in the layout of the patterns. These patterns show small depth of focus (DOF) and/or poor mask fidelity. The phenomena are analyzed by beam c onfigurations in interference and calculated optical images. For some patte rns, the defocus characteristics are essentially poor even in non-aberrated optics. For some other patterns, the spherical aberration degrades the cha racteristics significantly, while non-aberrated characteristics are not so poor. In the application of alternating PSMs for device pattern formation, it is important to consider the pattern layout and lens aberration.