Experimental Demonstration of Conjugate Structured Illumination Microscopy (c-SIM) for Sensing Deep Subwavelength Perturbations in Background Nanopatterns

The localization and classification of deep-subwavelength objects embedded in dense background nanopatterns in an imaging mode are challenging because of the optical diffraction limit and the weak signal-to-noise ratio and contrast. In this work, we, for the first time, experimentally validated the proposed conjugate structured illumination microscopy (c-SIM), which utilizes optical proximity correction techniques to generate a wide-field, diffraction-limited, and structured illumination field on the sample surface for defect inspection. Our experiments validated that c-SIM could accurately inspect 29 nm wide defects with an enhanced resolution (half of the diffraction barrier) using a 423 nm laser source. Moreover, our investigation demonstrated that different types of 38 nm wide defects could be precisely pinpointed and directly classified from the captured frames in the lateral scanning process, which is attributed to the fact that a conjugate structured light field could induce a high-intensity gradient in the illumination light. This technology may find diverse applications, such as a patterned wafer defect inspection, photomask inspection, material characterization, metamaterial inspection, and nanosensing.