Characterization of Microstructure, Interfacial Reaction and Diffusion of Immiscible Cu(Ta) Alloy Thin Film on SiO[sub 2] at Elevated Temperature

Abstract

Thin films of pure Cu along with Cu alloys with 0.1 or 2.3 atom % Ta were deposited on SiO 2 -covered Si substrates and subsequently annealed at 500-800°C in a vacuum. X-ray diffraction indicates that as compared to the pure Cu sample, the face-centered cubic Cu͑111͒ interplanar spacing increased in the as-deposited Cu ͑2.3 atom % Ta͒ sample because the Ta additives dissolved in the Cu films, indicating a nonequilbrium, super-saturated Cu͑Ta͒ solid solution formed by cosputtering deposition. After annealing, the resistivity of all samples was reduced and the lattice spacing of Cu͑Ta͒ films was restored to that of pure Cu film. Scanning electron microscopy shows that the surface of the Cu ͑2.3 atom % Ta͒ system remained smooth, and transmission electron microscopy reveals that the degree of Cu grain growth was very small after annealing. Rutherford backscattering spectrometry indicates that the Cu͑Ta͒ samples exhibited less diffusion of Cu into SiO 2 after annealing at 700°C, which can be attributed to the formation of a Ta 2 O 5 ͑or TaO x ) layer at the Cu(Ta)/SiO 2 interface, as revealed by X-ray photoelectron spectroscopy analysis. The connections between the Ta alloying contents and the microstructural as well as the interfacial characteristics of Cu on SiO 2 are discussed.

Four-zone solidification microstructure formed by laser melting of copper thin films

“…Energies of formation for Cu 2 O and CuO are 166.8 and 155.3 kJ/mol [50], significantly smaller than that for pure SiO 2 , suggesting that little oxygen may be available for copper oxide formation at this interface. In other experiments, it is observed that Cu can diffuse into SiO 2 at elevated temperatures, but the Cu-SiO 2 interface is non-reactive and non-wetting [51]. The films studied here are maintained at ambient temperatures at all times except during the laser irradiation, after which they quench back to ambient temperatures within 1 ms.…”

High-temperature self-grown ZrO2 layer against Cu diffusion at Cu(2.5at.%Zr)∕SiO2 interface

“…% Zr͒ samples, before and after annealing at 600-800°C. 10 After annealing, a very weak additional diffraction peak matching tetragonal ZrO 2 15 is observed in the Cu͑2.5 at. However, by enlarging the XRD patterns of Fig.…”

Correlation between outward diffusion of additives and oxidation of Cu(3.9at.%Ti) and Cu(2.3at.%Ta) thin films

“…From our previous Rutherford backscattering spectrometry result, 13,14 we realize that the Cu signal in the spectra of 700°C annealed Cu͑3.9 at. % Ti͒ sample is shifted slightly to lower energies, but that of 700°C annealed Cu͑2.3 at.…”

“…The accumulated additives on the free surface should be in form of metal oxides ͑TiO x or TaO x ͒, as examined by x-ray photoelectron spectroscopy reported in our previous study. 13,14 This oxide layer passivates the film surface and consequently retards the oxidation of Cu. Finally, the superior oxidation resistance of the preannealed Cu͑3.9 at.…”