RapidNano: Towards 20nm Particle Detection on EUV Mask Blanks

Abstract

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.

Deep sub-wavelength metrology for advanced defect classification

“…Figure 2 displays two pictures of TNO's Rapid Nano (RN3) particle scanner for blank substrates is based on dark field imaging 2,7 . The resulting signal is an image of the local scatter intensity for the uniformly illuminated field.…”

“…TNO's RN3 particle scanner (see Figure 2) has been developed to detect particle defects down to 42 nm in diameter on flat un-patterned uniform substrates up to 150 mm in lateral size, such as e.g. 100 mm XXX-flat Si wafers or EUV mask blanks 2 . This extreme level of sensitivity is achieved by illuminating the sample from multiple angles, in a variant of dark-field microscopy.…”

3D microscopy in industrial measurements

“…121 Dark-field scattering is widely used in automatic quantitative detection of surface defects. 122,123 At Zhejiang University, an evaluation system was successfully developed for a set of engineering surface defects using large-aperture and high-precision optical components. 124,125 The system can detect and evaluate surface defects in 850 mm × 500 mm optical components, with the minimum detectable scratch defect width of 0.5 μm and a microscale detection accuracy.…”