Speckles Reduction for an Aerial Inspection Tool, Laser Illuminator[1].

The brightness requirements for deep UV inspection systems necessitate choosing a laser as the light source. The monochromaticity and coherence of laser light, which are advantageus in other applications, bring about severe technical challenges when applied to microscopy. The requirement for large Étendue illumination (the product of illumination source area and cone angle) can be fulfilled only by using diffusers, placed in the illumination path. A Laser illuminator designed in this way will have the desired Étendue, but will be highly speckled with close to 100% speckle contrast. A popular speckle reduction technique is to use a rotating diffuser, which averages a large amount of speckles realization during the image integration time. It reduces the speckles contrast as N, N being the number of different realizations. However, for a sub-200nm systems, were an excimer Q-duration, this approach is impractical. Another way to achieve the speckle averaging is to use a multimode laser. If it can be implemented such that each mode will generate an uncorrelated speckle image; thus, 10K modes are needed to get ~1% speckle contrast, which is good enough for inspection systems. This work investigates the details of how to design a laser illumination system, such that most of its radiation modes will contribute to speckle reduction. The transverse radiation modes contribute mainly to the spatial coherence. It can be thought of as if the light shaping diffuser is illuminated with beams at different angles, each beam generating an uncorrelated speckle pattern, which does not interfere with speckle patterns due to other beams, resulting in a reduced speckle contrast. The longitudinal modes, on the other hand, will generate the same speckle patterns, and will not readily contribute to speckle reduction. In order to further reduce speckles, a special arrangement which takes advantage of the limited temporal coherence of the light can be used. This results in additional uncorrelated speckles realizations which contribute to the speckle reduction task. The overall result gives less than 0.5% speckle contrast, provided that the laser bandwidth is about 0.5 nm. result gives less than 0.5% speckle contrast, but with Laser bandwidth about 0.5 nm.

References: 1. R. Naftali, A. Guetta, H. Feldman, D. Shoham, US Patent No. “US20100097680 A1”.