Abstract
This paper presents a novel TTL (through the lens) alignment method, called SMART (separated marks TTL alignment), for optical lithography system. This alignment optical system could be accomplished without any complicated chromatic aberration compensation mechanisms. It was clarified that the relative displacement between the reticle and the wafer can be directly detected by measuring the intensity change from detector or the phase difference in the optical heterodyne beat signals. It is also demonstrated that the alignment signal is independent from the wafer focus variation, and can always be detected, even during exposure. An alignment signal stabilization method, in which part of the diffraction lights from the reticle marks are used as reference beat signal, has been adapted to obtain high positioning accuracy. As a result, alignment signal variation of less than ± 12 nm (0.012 μm) has been obtained.
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