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 () 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 SiO₂ (glass) and Si substrates. IR reflectivity was measured at 9.2 μm, while was measured with the help of laser ellipsometer at a wavelength of 632.8 nm. Two types of oscillations on R(d) and (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 (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 (d) with an oscillating period of 0.2 nm. For thickness larger than 12 nm all the oscillations tend to disappear and R and 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*.