| Summary: | 博士 === 國立中央大學 === 光電科學研究所 === 88 === This dissertation presents an original method for the measurement of the stress and the thermal expansion coefficient of optical thin films. The measuring method based on the phase shifting interferometry technique and five-step phase reduction algorithm. A circular disk polished on one side was used as the coated substrate during film deposition. The average stress in thin films can be derived by comparing the deflection of the substrate before and after film deposition. The deflection of the substrate by the deposited film was obtained by the phase map. Wavefront aberrations from tilt and defocus should be removed by fitting the Zernike polynomials. Using the Zernike polynomial fitting algorithm, a three-dimensional contour map and two-dimensional stress distribution were generated from the polynomial coefficients to visualize the deformation of the thin film and to examine the tensile or compressive stress after film deposition. The proposed method can facilitate the development of processes that yield more uniform films. Compared with other techniques, the present method is relatively simple, convenient and more accurate.
To investigate the mechanical properties of metal oxide films, five oxide films were prepared by ion-beam sputter deposition (IBSD). This dissertation also reports on the application of the phase shifting interferometry technique for the concurrent measurement of the thermal expansion coefficient and the elastic modulus of optical thin films. The stresses in metal oxide films were measured with the phase shifting interferometry technique using two types of circular disks with known thermal expansion coefficients, Young’s moduli and Poission’s ratios. The temperature-dependent stress behavior of metal oxide films was obtained by heating samples in the range from room temperature to 70 °C. The internal stresses of optical thin films deposited on the BK-7 and Pyrex glass substrates were plotted against the stress measurement temperature, showing a linear dependence. The slope of the line for the difference data was determined by least squares fitting. From the slopes of the two lines in the stress versus temperature plot, the intrinsic stress, the thermal expansion coefficient and the elastic modulus of thin oxide films are then determined. The experiment results of the stress behavior in metal oxide films were given. Finally, the database of the stress and the thermal expansion coefficient of optical thin films were established. It can provide the optimum deposition parameters to manufacture subnanometer bandwidth interference filters used for fiber-optic communication.
A novel measurement system for determining stress and thermal expansion coefficient of optical thin films has been successfully developed and constructed. It has the advantages of inherently high sensitivity, accuracy, easy operation and reproducible performance. In particular, it can be extended to varying-temperature applications and very useful in determining the mechanical properties of optical thin films.
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