Preliminary study of photomask pattern inspection by beam-shaped knife-edge interferometry

Abstract

This paper presents a photomask inspection and defect analysis technique to accurately measure the pattern dimensions and to detect the defects of the photomask in a convenient, fast, reliable manner. The beam-shaped knife-edge interferometry (BSKEI) was developed to collect the interferogram created by the superposition of the reference incident wave and the edge-diffracted wave while scanning the photomask. BSKEI consists of a pulsed laser diode, an aperture, an objective lens, and a fiber-coupled photodiode. The objective lens enabled the beam shaping to create a spherical wave as a new source of light, and the beam shaping was effective to enhance the spatial frequency of the interferogram. The interferograms were characterized based on a Fresnel number model, and the corresponding interferogram analysis methods were developed. BSKEI produced different interferograms according to the photomask patterns and defects while scanning the photomask. As a result, BSKEI is capable of the measurement of pattern width, opening width, line-edge quality, and opening area surface quality of the photomask, and is expected to be an alternative tool for critical dimension metrology in many semiconductor applications.