Calculation of optical constants from reflected spectral amplitude with Genetic Algorithm by white light interferometer

• Main Authors: Yi-sheng Siao, 蕭義勝
• Other Authors: Cheng-Chung Lee
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/23083570758383554020

碩士 === 國立中央大學 === 光電科學研究所 === 99 === The genetic algorithm (GA) is excellent at solving global numerical optimization problems. We propose GA to acquire optical constant of thin films from spectral amplitude retrieved using Fourier transform of the Interferogram. The optical parameters such as refractive index and thickness of thin films are essential for comparison of samples which are produced using different methods. These optical parameters are usually determined by photometric methods or Ellipsometer. The optical thickness, the product of refractive index and thickness determine the spectrum of the thin films. As optical constant in multiple of quarter wave we can find spectrum peaks. Under the condition when the single layer sample has larger optical thickness, we can acquire the optical parameters accurately through GA. In this study, we select five samples which are different materials and thickness and use GA to fit the spectrum to acquire the optical parameters of each sample. We use GA to fit the spectrum 1000 times and use the average optical parameters to calculate the spectrum comparing with the measured spectrum. We use three material Ta2O5, Nb2O5 and SiO2 as the samples. The refractive index of Ta2O5 and Nb2O5 are higher than SiO2, so it is easy for GA to acquire the optical constants under a wide range of the optical parameters. For Ta2O5 and Nb2O5, we set the range of refractive index between one to three and the thickness between 1 nm to 1000 nm. For SiO2, we change the range of refractive index between 1.4 to 1.5. The minimum errors of the calculated refractive index of Ta2O5 and Nb2O5 are smaller than 2% and the thickness are smaller than 1%. The total errors of the optical parameters are smaller than 5%. The errors of the calculated refractive index of SiO2 are smaller than 1% and the thickness are smaller than 3%. The same thin film sample would have different optical parameters on its two-dimensional surface. The Ellipsometer only measure one point on the thin film at a time and it cost at least thirty minute in a process. In this study, we use white-light interferometer with GA to measure the two-dimensional optical parameters on the thin film surface in a short time.