From Event: SPIE Optical Engineering + Applications, 2016
Traditionally, (dark field) imaging based, particle detection systems rely on identifying a particle based on its irradiance.
It can be shown that for a very smooth wafer with 0.1 nm surface roughness (rms) this approach results in a particle
detection limit larger than 20 nm. By carefully studying the physical mechanism behind this practical limit, we have
developed an alternative interferometric measurement technique that is able to improve upon this limit. This technique is
based on the interferometric amplification of the particle signal, while choosing the phase of the reference beam carefully
as not to amplify the coherent background speckle. Although this allows to detect particles that are 30% smaller,
compared to irradiance based detection this technique poses much more stringent requirements on the wavefront errors
of the imaging optics.
Wouter D. Koek, Erwin J. van Zwet, and Hamed Sadeghian, "Nano particle and defect detection: physical limit of state-of-the-art systems and novel measurement technique to improve upon this detection limit," Proc. SPIE 9960, Interferometry XVIII, 99600E (Presented at SPIE Optical Engineering + Applications: August 31, 2016; Published: 28 August 2016); https://doi.org/10.1117/12.2239885.
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