The background in simple dark field particle inspection shows a high scatter variance which cannot be distinguished from signals by small particles. According to our models, illumination from different azimuths can reduce the background variance. A multi-azimuth illumination has been successfully integrated on the Rapid Nano particle scanner. This illumination method reduces the variance of the background scattering on substrate roughness. It allows for a lower setting of the detection threshold, resulting in a more sensitive inspection system. By implementing this system the lower detection limit of the scanner was reduced from 59 nm to 42 nm LSE. A next improvement, a change of the inspection wavelength to 193 nm will bring the detection limit to sub 20 nm. © 2014 SPIE.