Characterization of EUV image fading induced by overlay corrections using pattern shift response metrology
The effects of EUV scanner actuated overlay corrections on image fidelity are discussed. Intrafield overlay corrections are implemented by reticle and/or wafer stage modulations during the exposure scan, which may lead to stage desynchronizations. The impact of such a mismatch on imaging is comparable to stage vibrations, which contribute to image blurring commonly known as image fading. For this study, deliberate stage desynchronizations were introduced by means of an asymmetric image rotation and effects on image fidelity qualitatively evaluated by pattern shift response (PSR) metrology. The PSR targets studied are blossom-style marks with asymmetric petal designs that transform process variations to a virtual pattern shift, which can be measured by conventional image-based overlay metrology. Corroborating as well as quantitative results were acquired by analyses of line width roughness. It was found that stage desynchronizations induced by overlay corrections can significantly degrade image fidelity starting with increased line width roughness up to a total pattern failure of linewidths relevant to current and future technology nodes. PSR metrology shows excellent capabilities to characterize relative image fidelity as well as across slit distortions and is therefore a suitable monitoring technique for on-wafer performance.