Differences in pattern displacement error under different illumination conditions

Off-axis illumination (OAI) technique is one of the most widely used resolution enhancement methods for sub 0.2 micrometer resolution in KrF lithography. Repeated patterns in DRAM drove many applications of OAI technique, such as annular, quadruple. There are optimum illumination shapes depending on pattern shapes and pitches. We measured pattern displacement error differences under two types of illumination shapes using box-and-box type overlay keys and real patterns to which we optimized illumination shapes. We focused on the differences of pattern displacement error between two pattern sets rather than pattern displacement error itself. The results show huge differences of overlay readings under various strong OAI settings. Finally, we suggested applying correction tables calculated by simulation with aberration data.