Extending depth of field for dark-field wafer surface inspection systems using digital holography

Dark-field imaging systems demonstrate significant advantages in the field of unpatterned wafer front-end defect inspection attributed to their high sensitivity, low cost, and high throughput. However, as optical systems with large numerical apertures (NA) are applied and inspection wavelengths gradually extend into the ultraviolet range, optical systems become extremely sensitive to defocusing. Surface variations on the wafer in the order of micrometers can cause complete defocusing in optical systems, leading to false positives and false negatives. To address this issue, this paper introduces a depth of field (DOF) extension method for dark-field inspection systems based on digital holography. Digital refocusing techniques is used to calculate the image stack at different focal depths based on the scattered light wave-front reconstructed from the hologram, and a focus measurement function evaluates each image to determine the optimal focal position. Finally, images near the optimal focal point are fused to create an all-in-focus image containing defect information on the wafer surface. The comparison between the processed results with the well-focused portions of the real captured images shows that the method accurately restores the distribution of defects on the wafer surface. The post-processing method extends the DOF of the optical system without compromising scanning efficiency, providing a solution to the defocusing issue in dark-field inspection systems for wafers.