Area-framing optical defect review under optical resolution using multi-NA dark-field microscopy images

Metrology and inspection (MI) processes are established at critical points of the semiconductor manufacturing process in order to maintain a certain yield and also provide information needed for future processes improvements. Typically, the inspection consists of dark-field (DF) inspection and SEM review/classification processes. An optical DF microscopy system (or inspection tool) first detects particles or pattern defects on wafers and obtains their position coordinates. However, due to its limited optical resolution, the DF system is not widely applied in the review process, which requires higher resolution images of the detected defects such as those provided by scanning electron microscopy (SEM) but with a sacrifice of throughput. We propose an innovative idea of applying two (or three) dark-field microscopy images for intermediate defect classification and size estimation under optical resolution. The proposed method utilizes the angular scattering distribution from a defect that is in the Mie scattering domain, which varies depending on both the beam and defect properties (wavelength, polarization, incident angle; shape, size, complex refractive index). It captures three dark-field images of the same wafer by three inter-changeable objectives with different magnification and numerical aperture (NA) values under identical side illumination conditions. We estimate the defect types and sizes simply by investing three measurements. We demonstrated this proposed method to classify and estimate the defect size down to similar to 80nm by an existing UV inspection tool with three DF imaging modes; 1) M15 mode, sampling stance = 150 nm, NA =0.6, 2) M25 mode, sampling distance = 250 nm, NA=0.36, and 3) M40 mode, sampling distance =400 nm, and NA =0.23. We demonstrated its feasibility by an independent SEM measurement of the detected defects.