The optical properties of evaporated metal films

This work in concerned with the investigation of the optical constants of metal films formed by evaporation under vacuum. Two methods of optical investigation were employed. In the first method the equations of Drude were applied to polarimetric measurements of the relative phase change and relative amplitude reduction of a light beam reflected from the surface of an opaque metal film at varying angles of incidence. Since most metals rapidly form an oxide film on exposure to the atmosphere a more complicated form of the Drude theory was applied to determine the optical cxonstants of the clean metal film. A vacuum polarizing spectrometer was constructed which measured films without breaking the vacuum of formation. Results obtained from the vacuum polarizing spectrometer were used as guide in performing the calculation of optical constants. These calculations were performed with the aid of digital computer. The second Method of optical investigation was based on that of Male and involved and measurement of the transmittance and reflectances of films using light at normal incidence. Results obtained from these investigations determined the variation of the refractive index of chromium and copper films with changing thickness. The existence of a relationship, the Wolter Relationship, beetwen the transmittance and reflectances for various metal films was verified. Electron microscopy and electron diffraction were employed to determine the evolution of the structure of films of several metals as the film thickness varied. A model based on that due to David which regard a thin metal film as being composed of small ellipsoidal particles was successfully applied to the results obtained for chromium and copper films. A comparison of the results obtained for the optical constants of chromium and copper films by the two methods, viz. (a) polarimetry involving oblique incidence of light and (b) reflectance and transmittance measurements involving only normal incidence of light, showed a wide discrepancy. A possible explanation is suggested which regards with caution results obtained by methods involving oblique incidence as it appears that metal films are not homogeneous isotropic uniform layers