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Image-based alignment sensor unaffected by aberration

Accurate alignment sensors are needed for tight overlay requirements. The interplay of alignment sensor lens aberrations and process variations (such as layer thickness variation or line width variation) can produce position measurement errors. We propose a new method called TIAS (two-beam imaging alignment sensor) where we can achieve high accuracy even in the presence of lens aberrations. It has added benefits of high image contrast and greater depth of focus. We confirmed the aberration robustness of TIAS by our experiment. TIAS technique is not only applicable to alignment sensing but also to image-based overlay (IBO) measurement.