Structured-illumination phase-derivative method for sub-diffraction-limit defect inspection

Optical defect inspection is fundamentally constrained by diffraction and by the weak response of intensity-based methods to defects that perturb the optical phase rather than the amplitude of the reflected field. We present a structured-illumination phase-derivative (SIPD) method that is sensitive to phase and identifies defects through the symmetry breaking induced in the reconstructed phase signal. Coherent fringe illumination shifts high-spatial-frequency defect information into the optical passband, and Fourier analysis of a phase-shifting sequence yields a complex field whose phase equals the difference between the two asymmetrically filtered channels. This phase difference varies slowly and smoothly across defect-free regions, whereas a localized defect introduces a sharp anomaly that is selectively enhanced by successive phase derivatives. Using a 0.4 NA objective at 532 nm, we localize defects as small as 100 nm, well below the Rayleigh limit, with a signal-to-noise ratio higher than that of conventional bright-field illumination. The method provides a practical, high-sensitivity route for defect detection and quality control.