## 3 Conclusions  
The main objective of this study is to improve the quality of the copper seed layer. The low step coverage caused by atoms accumulating at the top sidewall can eventually be improved by the chemical-mechanical planarization (CMP) process. However, the poor uniformity caused by atoms accumulating at the top sidewall can only be improved by re-sputter or thermal reflow, which is less effective than CMP at the wing. Thus, uniformity is considered as the priority goal in this study. From the results and discussion, remarks for depositing the copper seed layer for the interconnect metallization process are as follows:  
* The additional energy of atoms, such as increasing substrate temperature and increasing incident energy, leads the atoms at the wing to flow into the trench and mainly accumulate at the top sidewall. This increases the step coverage but decreases the uniformity. * Decreasing the deposition rate results in a better diffusion of deposited atoms to the substrate, thus increasing the effect of Ostwald ripening, which leads to better uniformity. * Different sections of the trench have optimal incident angles for receiving atoms, which are dependent on the size and features of the trench. Figure 37: Bottom height and necking with different substrate materials. Figure 38: Alloy percentage with different substrate materials. * The necking of the copper seed layer can be increased by decreasing the substrate temperature, incident energy, incident angle, and deposition rate. * The overall bottom height of the copper seed layer can be increased by increasing the incident energy and decreasing the substrate temperature, incident angle, and deposition rate. * The alloy percentage can be decreased by decreasing the substrate temperature, incident energy, and deposition rate. * Surface diffusion barrier energy caused by the potential energy and lattice structure is the dominant factor of different crystal planes. A lower diffusion barrier energy at the sidewall direction of the trench with a larger diffusion barrier energy at the wing direction of the trench allows the atoms at the top sidewall to flow to the wing, while stopping atoms from flowing the other way. This increases the necking and leads to better uniformity. * For different liner materials, the alloy percentage of the copper seed layer is the dominant factor. With greater cohesive energy, the alloy percentage decreases and thus decreases the additional diffusion barrier energy from the alloy atoms, which results in atoms at the top sidewall flowing towards the wing. Consequently, the necking and uniformity are increased. * The wing and top sidewall are the dominant areas of the copper seed layer. If the atoms tend to flow from the top sidewall to the wing rather than the other way around, necking would increase. Atoms depositing into the trench would thus increase and result in better uniformity. * Among the parameters discussed in this study, the diffusion barrier of different crystal planes is the most decisive factor, which leads to good uniformity. In general, optimization of process parameters and material properties requires online tuning to obtain the best quality copper seed layer under different circumstances. Based on the obtained results, valuable trend charts are developed for tuning recipe while depositing the copper seed layer. Crucial characteristics for evaluating the copper seed layer regarding different process parameters are included in the trend charts shown in Figures 39-41, where up and down arrows denote positive and negative correlations, respectively, and the number of arrows indicates the strength of the correlation. It should be noted that the trends shown in the results are not always linear and might differ under different conditions. Figure 39: Trend chart of sputtering process parameters and step coverage. **Author Contributions:** Conceptualization, C.-H.H., C.-K.C. and C.-L.C. ; methodology, C.-K.C. ; software, C.-H.H. ; validation, C.-L.C. and C.-K.C. ; formal analysis, C.-H.H. ; data curation, C.-H.H.