Multiple light scattering techniques are intensively investigated as potential characterization tools for a broad range of applications. We are reporting on the noninvasive characterization of filters used in processes such as slurries filtering for CMP.
Filters are soft porous membranes characterized by their pore size distribution and thickness, and a noncontact, nondestructive optical procedure to measure these properties is highly desirable. Due to their internal inhomogeneity, porous media strongly scatter light and, therefore, a specific procedure needs to be developed.
In this work, low coherence interferometry is used to investigate light propagation in the filter and obtain the reflectivity as a function of optical pathlength for backscattered photons. This can be subsequently related to the optical properties of the sample using analytical and/or numerical models, and the porosity of the sample can be determined. In the case of filters with thicknesses much larger than the wavelength, a diffusion approximation for light propagation is used to infer the porosity information. For thinner membranes, numerical methods are used to describe the intermediate low-scattering regime that can not be represented analytically. As a direct result of the measurement, the thickness of the filter is determined independent of porosity.