Sub 20nm particle inspection on EUV mask blanks

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.