The inspection of particles plays an important role in quality control of smooth surfaces, such as optics and silicon wafers. The difficulty of inspection lies not only in the high-resolution detectivity for small-size particles, but also in the need to distinguish among different types of particles due to their different damages and re-work processes. This paper proposes a detection and discrimination method of the particles on (surface particles) and below smooth surfaces (subsurface particles) by laser scattering with polarization measurement. The incident laser beam passes through a polarization state generator (PSG) to illuminate the smooth surface. A large proportion of the particle-induced scattered light is detected after collected by an ellipsoidal mirror. Meanwhile, in the blind area of the ellipsoidal mirror, a small amount of scattered light enters the polarization measurement channel, where a polarization state analyzer (PSA) is used to finally obtain a two-dimensional polarization signal for discriminating the two types of particles. Then, by a nonlinear global search combined with the theoretical models for polarized light scattering from particles, the optimal polarization measurement state of the PSG and the PSA is obtained to maximize the separability between the surface particles and subsurface particles on the acquired polarization signals. This method takes full advantage of the polarization of scattered light, and with one single scan the inspection for the whole surface can be accomplished. Finally, experimental results demonstrate the effectiveness and accuracy of the method.

Date Published: 3 September 2019
PDF: 10 pages
Proc. SPIE 11102, Applied Optical Metrology III, 111021F (3 September 2019); doi: 10.1117/12.2528373

Published in SPIE Proceedings Vol. 11102:
Applied Optical Metrology III
Erik Novak; James D. Trolinger, Editor(s)