Annealing temperature dependence of optical and structural properties of Cu films

Annealing temperature dependency of the optical and structural properties of Cu films is carefully studied using multiple means such as spectroscopic ellipsometry, x-ray diffraction, white-light interferometry, atomic force microscopy, scanning electron microscope (SEM), and the four-probe method. A wide annealing temperature range from 300 to 1200 K is examined experimentally. We find that the optical and structural properties of Cu films annealed at above 600 K cannot be naturally extended from the data commonly achieved on the samples annealed at the temperature below 600 K. When the annealing temperature is increasing, two transition points at around 600 and 900 K have been found by the Drude-Tauc-Lorentz analysis of the dielectric functions covering the energy range from 0.73 to 6.42 eV. The transition feature at 600 K can be attributed to the improvement of the crystalline state caused by annealing, which can be interpreted by the sharper diffraction peak shown in x-ray diffraction (XRD). The transition feature at 900 K was caused by the emergence of island structure due to the high-temperature annealing, which was supported by the SEM images as well. In fact, both the conspicuous scattering characteristics in the pseudodielectric function and the spurious peaks in the XRD pattern indicate the dramatically morphologic changes induced by the migration and reaggregation of Cu nanoparticles. The downward trend of dc resistivity captured in dielectric function analysis was supported by that of the sheet resistivity measured by the four-probe method.