Surface profile measurement by using a focused-beam optical probe

Abstract

This paper proposes a novel scanning deflectometry method using a confocal probe based on second harmonic generation (SHG) to achieve both high lateral resolution and a wide angular measurement range. In recent advanced manufacturing, there is an increasing demand for measuring high-curvature optical elements, such as X-ray mirrors, with sub-nanometer height precision and micrometer-scale lateral resolution. However, conventional scanning deflectometry faces a trade-off between angular resolution and measurement range, often requiring complex specimen rotation on high-precision stages. Furthermore, while focused-beam methods improve lateral resolution, they typically suffer from limited angular ranges and stringent focal alignment requirements. To address these challenges, we introduce a concept utilizing a so-called SHG-based confocal probe as a measurement tool for scanning deflectometry. By exploiting the phase-matching conditions in birefringent crystals, this probe achieves a wide angular measurement range of several hundred milliradians while maintaining high angular resolution. Additionally, the focusing properties within the crystal enable auto-focusing capabilities and a high lateral resolution. Preliminary evaluations indicate a lateral resolution of 18.62 μm, which, combined with an angular resolution of 5 arcseconds (approximately 24 μrad), yields an estimated height resolution of approximately 450 pm. The proposed method offers a promising approach for the high-precision profile measurement of high-curvature surfaces without the need for complex mechanical rotation.