A characterization method of thin film parameters based on adaptive differential evolution algorithm (Invited)

According to the transmission mode of polarized light in Mueller ellipsometry, a characterization method for the thickness and optical constants of isotropic nano films based on Self-Adaptive Differential Evolution algorithm (SADE) was proposed. By establishing the least square model of the output light intensity with respect to the Mueller matrix of the standard sample to be measured, the elements of the Mueller matrix were solved by using the Sade algorithm, and the Mueller spectral curve obtained by fitting was compared with that measured by dual rotating-compensator Mueller matrix ellipsometer (DRC-MME), and the film thickness was calculated by using the transfer matrix. The SiO₂/Si standard samples with calibration values of (104.2±0.4) nm and (398.4±0.4) nm were simulated and calculated. The experiment shows that: when numbers of iterations accumulated to 80 and 87 respectively, the residual square sum of the light intensity of the objective function converges to the minimum values of 0.97 and 1.01. The calculated film thickness values are (103.8±0.6) nm and (397.8±0.6) nm respectively, and the relative errors are both less than 1%. At the same time, a metrological ellipsometer was used to calculate according to the obtained refractive index, and the calculated values of the film thickness were 104.1 nm and 398.2 nm. It is verified that SADE has the characteristics of simple calculation and accurate global optimal solution in solving the parameters of isotropic nano films at a similar convergence rate.