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.

Figure 1

Fitting results of ψ and Δ for Cu films annealed at temperature from 300 to 1200 K at the incident angle of 60°. (a), (b) Fitting results of ψ and Δ at temperature from 300 K, respectively. (c), (d) Fitting results of ψ and Δ at temperature from 500 K, respectively. (e), (f) Fitting results of ψ and Δ at temperature from 600 K, respectively. (g), (h) Fitting results of ψ and Δ at temperature from 800 K, respectively. (i), (j) Fitting results of ψ and Δ at temperature from 900 K, respectively. (k), (l) Fitting results of ψ and Δ at temperature from 1000 to 1200 K, respectively. Circles represent the measurement values, while solid lines stand for the calculated values.

Figure 2

Fitting results of ψ and Δ for Cu films annealed at temperature from 300 to 900 K at the incident angle of 60°. (a) Fitting results of ψ; (b) Fitting results of Δ. Circles represent the measurement values, while solid lines stand for the calculated values. Both ψ and Δ exhibit a transition temperature at 600 K, at which the variation tendency of both ψ and Δ will change.

Figure 3

Pseudodielectric functions $\langle \varepsilon \rangle$ of annealed Cu films. (a) The real part $\langle {\varepsilon }_1\rangle$ and (b) the imaginary part $\langle {\varepsilon }_2\rangle$ corresponds to 300, 500, 600, 800, 900, 1000, and 1200 K. (c) The real part $\langle {\varepsilon }_1\rangle$ and (d) the imaginary part $\langle {\varepsilon }_2\rangle$ of Cu films annealed at 800 and 1000 K at the incident angle of 60°, 65°, and 70°. At the annealing temperature below 900 K, the angle dependency of $\langle \varepsilon \rangle$ can be ignored, while the angle dependency of $\langle \varepsilon \rangle$ is very apparent for the annealing temperature above 900 K.

Figure 4

Depolarization of annealed Cu films. (a) The temperature dependency of depolarization at incident angle of 60°. (b) The depolarization of Cu films annealed at 800 and 1000 K at the incident angle of 60°, 65°, and 70°. The depolarization of Cu films annealed at 1000, 1100, and 1200 K are shown in inset. At the annealing temperature below 900 K, the angle dependency of depolarization can be ignored, while the angle dependency of depolarization is very apparent for the annealing temperature above 900 K.

Figure 5

Dielectric functions ε of Cu films annealed at different temperature from 300 to 1200 K. The dielectric functions are calculated from ellipsometric parameters via Drude-Tauc-Lorentz analysis. The real parts of ε corresponding to temperature from 300 to 900 K are displayed in a logarithmic fashion. The dielectric functions ε of Cu films annealed at 1000, 1100, and 1200 K are shown in insets. Dielectric functions of bulk Cu at 300 K reported by Palik [31] (dark blue dashed line) and John and Christy [32] (dark red dashed line) are shown for comparison. At the annealing temperature below 900 K, the dielectric functions of each annealed Cu film have great similarity with the published data. Both of them show prominent metallicity. However, at the annealing temperature above 900 K, the plasmonic feature is predominant in the dielectric functions.

Figure 6

Surface morphology of Cu films annealed at 300, 600, 800, 900, and 1200 K reported by AFM. All the micrographs are shown at the same scale of 500 nm. With the annealing temperature increasing, the Cu particles coalesce gradually to larger clusters. At the annealing temperature above 900 K, the islands or hillock are formed and separated.

Figure 7

X-ray diffraction pattern of Cu films annealed at different temperature from 300 to 1200 K. All the diffraction patterns have three diffraction peaks at 43.4°, 50.5°, and 74.2°, corresponding to (111), (200), and (220) of Cu (ICSD-53247). With the annealing temperature increasing, both the 111, 200, and 220 peaks have exhibited obvious dynamic changes, which indicate the evolution of crystal structures in the Cu films.

Figure 8

The crystallite sizes in the (111), (200), and (220) plane obtained by XRD, the average crystallite size, and the thermal differential of average size. With the annealing temperature increasing from 300 to 1200 K, all the crystallite sizes exhibit the gradual upward trends.

Figure 9

The average lattice parameters obtained by XRD, thermal dynamics simulations, and annealing dynamics simulations. Both the thermal dynamics simulations and annealing dynamics simulations are carried out based on molecular dynamics.

Figure 10

Sectional view of Cu films annealed at different temperature. All the images are presented at the same scale and magnification. With increasing the annealing temperature, the Cu particles reaggregate on the lateral scale and eventually form island structure.

Figure 11

dc resistivity obtained from fitting procedures and layer resistivity measured via four-probe method.

Figure 12

Scattering time of conduction electrons at each annealed Cu film.

Figure 13

Absorption peak energy and relaxation time of conduction electrons at each annealed Cu film.