EVOLUTION OF THE OPTICAL FUNCTIONS OF THIN-FILM ALUMINUM - A REAL-TIME SPECTROSCOPIC ELLIPSOMETRY STUDY

We report a comprehensive study of the optical functions of thin-film aluminum from initial nucleation through continuous film growth. This study relies on ellipsometric spectra from 1.3 to 4.0 eV collected in real time with a multichannel instrument during both thermal evaporation and magnetron sputtering of Al onto SiO2 substrates at room temperature. The spectra for all films in the particle growth regime can be modeled with a Maxwell-Garnett-type effective medium theory, modified to include dipole interactions between spheroidal particles arranged on a square grid. The dielectric-functions of the Al particles themselves, as well as those of continuous films, are interpreted assuming variable relaxation times for both Drude and interband electronic contributions. The relaxation times are determined by a common value of the mean free path, reduced from its bulk film value owing to electron scattering at defects, particle surfaces, and grain boundaries. For all films in the particle regime of growth, i.e., for thicknesses < approximately 50 angstrom, the deduced relaxation times are independent of thickness (and hence particle size), and are more than an order of magnitude lower than bulk film values. This suggests a defective structure in which electron scattering at internal particle defects limits the relaxation time and determines the particle optical functions. For Al prepared by high-rate evaporation, a transition is observed at a thickness of 55-60 angstrom, just after continuous film formation. At the transition, the interband electron relaxation time increases abruptly, signaling the development of higher-quality crystalline grains that extend throughout the film thickness. Only after this transition is the (200) parallel-band absorption feature visible in the Al dielectric function at 1.5 eV. For thicknesses > 60 angstrom, the interband relaxation time increases with thickness, providing evidence that grain-boundary scattering is the dominant mechanism controlling the optical properties in the bulk film stage.