The Use of Ellipsometric Technique in the Study of Physical Adsorption and Low Energy Ion Bombardment

Abstract

The optical and electrical effects of gas adsorption on polycrystalline and single crystal gold films have been observed simultaneously. The bombardment of such films with low energy inert gas ions has been studied, in an attempt to relate ellipsometric parameters to the damaged layer. Gold and inert gases were chosen to eliminate the possibility of changes in the optical constants due to oxidation or any chemical reactions between substrate and ambient. Polycrystalline films were prepared on glass slides held at room temperature in an ultra-high vacuum system. Single crystals were prepared on sodium chloride crystals of <110> and <100> orientation held at 350-400°C and 2-6 x 10-9 torr. The optical constants of the polycrystalline gold films were determined in the wavelength range 0.39 - 1µ and compared with published results. The sensitivity of the gas adsorption measurements was investigated over a wide range of wavelengths for two gases, argon and neon, and was found to be higher at wavelengths corresponding to an emission line for the gas used. Gas adsorption was found to depend on the surface structure of the films, the amount of adsorption on polycrystalline films being approximately the average of those found for <110> and <100> orientations. Argon and neon were found to be optically absorbing at some wavelengths. Low energy ion bombardment (surface held at room temperature) gave similar effects to gas adsorption for ion energies below a critical value corresponding to the onset of damage. These critical ion beam energies were 100 eV for argon ions on <100> gold, 110 eV for the <110> surface and 130 eV in the case of polycrystalline gold. The thickness of the damaged layer was calculated from the results and found to be in reasonable agreement with the range computed from the stopping power of the material in accordance with the theory of Lindhard et al. (1963). Changes in residual resistivity, measured at 4°K confirmed the order of magnitude of damage layer thickness. The degree of damage was related to the surface structure, and ion type. Annealing experiments showed that the damage produced was not only simple Frenkel pairs, but also dislocations resulting primarily from injected interstitial atoms or from substrate atoms displaced by energetic ions.