A Novel Technique for Measuring Defocus with Phase Shift Gratings on a Photomask

Abstract

A focus monitor reticle having a novel grating structure is proposed. If a grating pattern has a property of being able to generate asymmetry between positive and negative diffraction orders, a defocus of the grating pattern can be translated to an easily measured pattern shift perpendicular to the optical axis. A possible grating for this purpose is composed of three components: opaque line, naked line and #x03C0;/2-phase-shifted groove. In particular, with their width ratio equivalent to 2/1/1, one of the two first-order beams disappears entirely. Consequently, tilted standing waves are formed by two-beam interference between zeroth-order and the other first-order on the image plane. This paper describes a new technique for measuring a defocus with a phase shift grating object. Good agreement between experimental sensitivities and theoretical expectation verifies that the technique achieves high accuracy of several nanometers or better. Applications of the technique show indicative ability in field curvature and astigmatism of the state-of-the-art scanner.