Ellipsometric determination of permittivity of silver nanolayers

Optical properties of ultra-smooth silver nanolayers fabricated with e-beam physical vapour deposition system on epi-polished silica and sapphire substrates are measured using spectroscopic ellipsometry method. Ag films of 100 nm thickness are deposited at 180 and 295 K directly on substrates as well as on intermediate 1 nm wetting layer of germanium. Different deposition rates are used for Ge (0.5 Å/s) and Ag (10 Å/s) films. Roughness of subsequent layers is measured with atomic force microscope and expressed in peak-to-peak and root-mean-square values. The Ge wetting layer considerably decreases roughness of Ag surface. Ellipsometric measurements made in the spectral range 193 nm - 2.2 μm (6.5 eV - 0.6 eV) for different incidence angles are interpreted using two-media model with interfaces air/Ag, Ag/silica or Ag/sapphire and three-media model with additional interfaces Ag/Ge and Ge/silica or Ge/sapphire. The RMS values of Ag surface roughness are used to model the intermix layer thickness at the air/Ag interface. The measurement spans a wide range of frequencies which are lower than the unscreened plasma frequency of bulk silver close to 9 eV. Plots of the imaginary part of permittivity of silver films versus photon energy are presented and discussed.