Monitoring the thickness of white Sn thin island films by spectroscopic ellipsometry

Spectroscopic ellipsometry of tin (Sn) thin island films on silicon (Si) is investigated to enable monitoring of the progress of deposition and etching. Since our applications only require a measure of the quantity of Sn, we consider here a fit model with low complexity. The trade-offs are high fit speed, the avoidance of complex input requirements, and broad compatibility with instrument software. The model describes the island film as an effective layer with a Lorentzian dielectric function, for which the effective (or “optical”) thickness is obtained. The key question in this paper is how this thickness can be interpreted. For experimental layers, the optical thicknesses are found to be linearly proportional to the actual tin mass thicknesses determined using x-ray fluorescence. The proportionality constant varies from 1.6 ± 0.2 for tin on hydrogenated silicon (Si:H) to 2.33 ± 0.11 on thermal SiO2. Analysis of data simulated with the Bedeaux–Vlieger model relates this major trend to differences in the island shape. Furthermore, the optical thickness from in situ data during a deposition shows a near-linear increase for hemispherical islands on Si:H, and a nonlinear increase for (nearly) full spheres on SiO2. The simulations relate this minor effect to island-island interactions that depend on island coverage and shape. It is concluded that the optical thickness is a usable measure to monitor the quantity of Sn on Si:H during a process. For Sn on SiO2, the model provides a more qualitative indication of whether growth or etching occurs.