New phase-shift gratings for measuring aberrations

Abstract

A new focus monitor reticle is proposed to measure field curvature and even wave-front aberrations. A grating pattern comprising opaque line, naked line and (pi) /2-phase-shifted groove with their width ratio equivalent to 2/1/1 makes either of first-order rays of diffraction disappear entirely. The other first-order ray interferes with the zeroth-order ray to form interference fringes with the ability of moving proportional to a defocus. This paper describes basic characteristics of the reticle and demonstration for a krypton fluoride (KrF) excimer laser scanner with a numerical aperture (NA) of 0.73 as an application of the reticle. Using overlapped exposures and an overlay inspection tool, the measurement of field curvature verified to achieve high accuracy of several nanometers or better. Besides, even wave-front aberrations up to six-foil were characterized with a high degree of accuracy.