The via structure is modeled as a film with an adjustable ratio of the illuminated surface areas. When illuminating beam spot moves across the via arrays target, the intensity of reflected light oscillates due to vias moving in and out of beam spot. Mata-Mendez and Chavez-Rivas^{23} have estimated that for grating with line-to-space ratio of $1\u22361$, the amplitude of oscillation becomes negligible when the minimal number of gratings inside the beam spot is two, which is corresponding to 50% of the illuminated surface areas. For the HDTSV array structure of few microns via size, the ratio of the illuminated area of the top silicon surface to the via’s opening within the illuminated area is considered to be constant, and it depends only on the via’s opening shape and pitch. The illumination spot covers a number of vias: most of the vias are entirely illuminated; only few of them under the spot edge are partially illuminated. Although illumination spot might slightly move with respect to array between repeated measurements, the influence on the total area of vias varies little, and therefore, the variation of intensity of reflected light can be neglected. Assuming a particular value for the ratio, the coefficients $\alpha $ and ($1\u2212\alpha $) are the portion of the illuminated top silicon surface and the via opening, respectively, $E0$ is the electrical field incident on the silicon surface and the via area, the via depth is $d$, and the wavelength is $\lambda $. The reflectance intensity $I\xaf$, which is the sum of the two reflected beams from the silicon surface and the via bottom surface, is Display Formula
$I\xaf=Const|\alpha E02+(1\u2212\alpha )E02+2\alpha (1\u2212\alpha )E02\u2009cos[2\pi (2d/\lambda )]|.$(1)