Cleanliness is a prerequisite for obtaining economically feasible yield levels in the semiconductor industry. For the next generation of lithographic equipment, EUV lithography, the size of yield-loss inducing particles for the masks will be smaller than 20 nm. Consequently, equipment for handling EUV masks should not add particles larger than 20 nm. Detection methods for 20 nm particles on large area surfaces are needed to qualify the equipment for cleanliness. Detection of 20 nm particles is extremely challenging, not only because of the particle size, but also because of the large surface area and limited available time.In 2002 TNO developed the RapidNano, a platform that is capable of detecting nanoparticles on flat substrates. Over the last decade, the smallest detectable particle size was decreased while the inspection rate was increased. This effort has led to a stable and affordable detection platform that is capable of inspecting the full surface of a mask blank.The core of RapidNano is a dark-field imaging technique. Every substrate type has a typical background characteristic, which strongly affects the size of the smallest detectable particle. The noise level is induced by the speckle generated by the surface roughness of the mask. The signal-to-noise ratio can be improved by illuminating the inspection area from nine different angles. This improvement was first shown on test bench level and then applied in the RapidNano3. The RapidNano3 is capable of detecting 42nm latex sphere equivalents (and larger) on silicon surfaces. RapidNano4, the next generation, will use 193 nm light and the same nine angle illumination mode. Camera sensitivity and available laser power determine the achievable throughput. Therefore, special care was given to the optical design, particularly the optical path. With RapidNano4, TNO will push the detection limit of defects on EUV blanks to below 20nm.

The Rapid Nano is a particle inspection system developed by TNO for the qualification of EUV reticle handling equipment. The detection principle of this system is dark-field microscopy. The performance of the system has been improved via model-based design. Through our model of the scattering process we identified two key components to improving the inspection sensitivity. The first component is to illuminate the substrate from multiple azimuth angles. The second component to improve the sensitivity is to decrease the wavelength of illumination. A shorter wavelength increases the total scattering and reduces the background scattering relative to the defect signal. A new Rapid Nano particle detection system (RN4) will be completed mid 2016. It combines the multi-azimuth illumination mode with a 193 nm source. This system will have a sub 20 nm LSE sensitivity, in-line with the requirements of the ITRS roadmap for defects on EUV masks. The Rapid Nano inspection system makes use of dark-field imaging, in which an area of a substrate is imaged on a camera. Previous generations of the Rapid Nano system made use of commercially available optics for the imaging step. In the DUV wavelength regime diffraction limited imaging over a large field is more challenging and suitable optics were not available off-the-shelf. Therefore TNO designed and fabricated an objective lens specifically for the Rapid Nano 4 inspection system. Other challenges in changing the illumination to the DUV include handling the high peak power of the pulsed laser source and the lifetime of the optics. The design of the Rapid Nano 4 and first results comparing it to the model predictions will be presented.