High-speed Coherent Fourier Scatterometry

Abstract

Coherent Fourier Scatterometry (CFS) is a powerful optical metrology technique for the precise characterisation of nanostructures. Conventional CFS systems rely on piezo-based scanning stages for raster scanning, which limits throughput due to slow scanning speeds. In this work, we present a high-speed CFS system incorporating a galvanometric (galvo) mirror for beam scanning. This approach significantly enhances scanning speed while maintaining measurement accuracy. Although galvo mirrors are widely used in optical systems, their implementation in CFS has unique challenges such as off-axis beam aberrations and angle-dependent beam shifts at the split detector. These issues are analysed and mitigated through optical design, alignment and system calibration. Additionally, we derive the minimum detector bandwidth required to capture high-frequency signals generated by the fast scanning. The effectiveness of the system is demonstrated through the calibration of pits with various diameters that are etched onto a silicon wafer. Results show a substantial improvement in scanning speed as compared with piezo-based systems without compromising measurement precision, making this approach highly suitable for high-throughput metrology applications.