The Anomaly of Periodicity Doubling in Projection Photolithography of Periodic Features

In the case of duplicating periodic features by UV projection lithography, the unwanted phenomenon of periodicity or frequency doubling somehow occurs due to far-field imaging of the near-field diffraction. In this letter, we fundamentally explored the mechanism of frequency or periodicity doubling in an analytical and numerical way, and reconfirm the validity by experiments of projection lithography. In this process, both the Talbot self-image and π-phase-shifted image were synchronously captured to generate the overlapped pattern with doubled periodicity within the depth-of-focus (DOF) area. The Fresnel diffraction theory was used to further derive the relationship between the mask and the objective lens' parameters (i.e., the resolution and DOF). Finally, experimental results showed the self-image and π-phase-shifted image, both clearly recorded on wafer when optical parameters were well adjusted.