Infrared reflectivity and dielectric permeability of ultra-thin Cu and Al films

In this report we present an experimental investigation of the reflectivity ( R) and the dielectric permeability ( ε) for Cu and Al ultra-thin films ranging in thickness from a few monolayers to 12 nm at infrared and visible wavelengths. The metal films were prepared by RF-sputtering on SiO 2 (glass) and Si substrates. IR reflectivity was measured at 9.2 μm, while ε was measured with the help of laser ellipsometer at a wavelength of 632.8 nm. Two types of oscillations on R( d) and ε( d) were discovered for two thickness regions determined by the critical thickness value d*. Oscillations at d< d* with periods near 0.3 nm for Al and Cu films were observed on R( d) and ε( d) due to quantum sized effects (QSEs). At d> d* (thickness between 6–12 nm) we discover a new type of strong oscillation of R( d) and ε( d) with an oscillating period of ∼0.2 nm. For thickness larger than 12 nm all the oscillations tend to disappear and R and ε behave almost as their volume values. A possible explanation for the appearance of these two kinds of oscillations is based on the introduction of the critical film thickness d*.