Impact of Reticle Topography on Field Curvature and Overlay Errors in Optical Lithography

An accurate method for measuring defocus, which is an application of phase shifting mask technology for higher resolution microlithography, reveals the impacts of reticle topography on optical microlithography. On the topographic reticles, there exist many focus monitors besides overlay marks. These reticles include almost flat and convex to concave topography with a maximum altitude difference of 2 µm. Contrary to our expectation, the experimental results showed that reticle topography has little impact on the field curvature, but has a significant impact on overlay errors. The lithography tool used in this experiment presses topographic reticles flatly against the face of a reticle stage to plane the field curvature, with generating magnification errors only in the slit direction.