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Linewidth control across an exposure field is becoming increasingly challenging as design rules shrink. Contributions to linewidth variation can arise from the reticle, the exposure tool and the resist process. For the exposure system, errors may originate from the illuminator ste-up, the projection lens aberrations using a new reticle and measurement technique. The technique uses a special reticle, which converts wavefront phase errors to displacements on the wafer. These offsets can be measured using conventional overlay tools with greater speed and accuracy than SEM measurements of small linewidths. Reconstruction of the wavefront using this data provides a more reliable in-situ characterization of aberrations.
Farrar et al. (Wed,) studied this question.