## 3 Results and Discussion
### Crystal Plane  
The dominant factor of different crystal planes is the diffusion barrier energy. Lower diffusion barrier energy increases the diffusivity of the atoms significantly. Various crystal  
Figure 28: Alloy percentage with different deposition rates.  
Figure 26: Uniformity with different deposition rates.  
Figure 27: Bottom height and necking with different deposition rates.  
planes on different trench surfaces were studied in this paper. The defined crystal planes studied in this paper are as follows:  
* (100): (100) at X direction, (010) at Y direction, (001) at Z direction.
* (110) x: (110) at X direction, (\(\overline{1}\)10) at Y direction, (001) at Z direction.
* (110) z: (001) at X direction, (1\(\overline{1}\)1,0) at Y direction, (110) at Z direction.
* (111) x: (111) at X direction, (\(\overline{2}\)11) at Y direction, (0\(\overline{1}\)1) at Z direction.
* (111) z: (0\(\overline{1}\)1) at X direction, (2\(\overline{1}\)1) at Y direction, (111) at Z direction.  
Figure 29 illustrates the copper seed layer with crystal planes. The step coverage and uniformity with different crystal planes are shown in Figures 30 and 31, respectively. Diffusion barrier energy is dependent on the atomic structure of the surface, and it varies between different materials and surface orientations. As shown in Table 2, the diffusion barrier energy of (110) x and (111) z was relatively low in the x direction, while the diffusion barrier energy was high in the z direction, which led to the best uniformity for crystal plane (110) x and (111) z. This caused atoms to diffuse from the top sidewall to the wing more easily than from the wing to the top sidewall. In other words, atoms were more likely to flow from the top sidewall to the wing, rather than the other way around. This decreased the number of atoms accumulating at the top sidewall significantly, and consequently this led to an increase of the necking, as revealed in Figure 32. With the great increase of necking, the number of atoms at sidewalls and the bottom of the trench increased sufficiently. These mechanisms increased the uniformity of the copper seed layer.  
Figure 30: Step coverage with different crystal planes.  
Figure 29: Copper seed layer with different crystal planes.  
The crystal plane of (111) x, however, had high diffusion barrier energy in the x direction and low diffusion barrier energy in the z direction. The low diffusion barrier energy at the wing increased the diffusivity of the atoms, allowing them to flow to the top sidewall, while the high diffusion barrier energy at the sidewall caused most of the atoms to accumulate at the top sidewall, forming huge clusters and decreasing the necking. With the decreased necking, atoms reaching the mid sidewall, lower sidewall, bottom center, and bottom side decreased, causing their uniformity to decrease. This crystal plane, however, led to relatively large step coverage, since the number of atoms at the wing decreased significantly.  
It was noticed that crystal planes with larger lattice spacing, such as (111) x and (111) z, allowed the depositing atoms to impact deeper into the substrate, thus increasing the alloy percentage, and vice versa, as shown in Figure 33. In general, this shows that a greater  
\begin{table}
\begin{tabular}{c c c} \hline
**Surface** & **Layers** & **Surface Diffusion Barrier Energy (ev)** \\ \hline Ta (100) & 16 & 0.883 \\ Ta (110) & 14 & 0.543 \\ Ta (111) & 14 & 2.076 \\ \hline \end{tabular}
\end{table}
Table 2: Diffusion barrier energy on different crystal planes of tantalum [13].  
Figure 31: Uniformity with different crystal planes.  
Figure 32: Bottom height and necking with different crystal planes.  
diffusion barrier energy at the wing and lower diffusion barrier energy at the sidewall lead to better uniformity. These mechanisms are caused by the difference of diffusion barrier energy on various crystal surfaces of the trench.