A novel optical technique uses a pulsed double cavity laser at 532 nm with beam-shaping optics that is fired at a synchronized time delay after the EUV burst. The light is converted to a thin light sheet that illuminates possible debris fragments in the direct region around the plasma.
Mie scattering theory is applied to convert the intensity of the incoming individual particles to an estimated diameter estimation and the two frames are correlated with advanced particle tracking algorithms to capture the velocity and direction of each individual particle. Because light intensity is used for particle sizing, small individual particles can be detected. The technique provides particle count, diameter, direction and velocity information.
This technique has successfully been applied on operating NXE test sources. It has proven to directly identify plasma conditions with significant debris reduction. Furthermore, it has potential to correlate the plasma settings to lifetime estimations and thus can be used for both source optimization and design.