Thermal Annealing and Impurities Incorporation of Electrodeposited Cobalt Thin Film
Authors: Hu Yang , Huang Qiang 
Published: February 2020

Cobalt has recently attracted much attention as it is replacing copper in the ground rule metal level in advanced interconnects to minimize the resistivity increase in small features1. Defect-free filling of Co in small features has been demonstrated using various commercial additives2-4. Annealing has been used to further decrease and stabilize the resistivity of lines5. It is well known from copper studies that additives in electrolytes will not only be incorporated into the film as impurities but also affect the grain structures and resistivity of deposited film6, 7. Post deposition annealing has been used to further grow the grains and to influence the resistivity and reliability of lines and vias8. In this report, we use several different additives including dimethylglyoxime (DMG), sodium chloride (NaCl), and sodium 3-mercapto-1-propanesulfonate (MPS-Na) to study their individual and synergistic effects on impurity incorporation (C, N, Cl, S) in cobalt thin films. The film morphology, resistivity, crystallographic structure, and the impurity distribution were also investigated.
Figures 1 (a) and (b) show the top-down SEM micrographs of two cobalt films deposited with different combinations of additives, one with 100 ppm DMG and 100 ppm MPS-Na, and the other with 100 ppm DMG and 100 ppm NaCl. A strong influence of additives on the morphology of Co films was observed. Figure 1 (c) shows the film sheet resistance change upon 300 C anneal in vacuum. The resistance decreases along the annealing time, but quickly settles within the first 2 hours except for the film deposited with a combination of the three additives (red line), 100 ppm DMG, 100 ppm NaCl, and 100 ppm MPS-Na. The later not only shows a much higher as deposited sheet resistance but also took a much longer 6 hours for the resistance to stabilize. Secondary ion mass spectroscopy (SIMS) was also used to quantify the impurities in the Co films. The resistivity change upon annealing will be discussed in detail in conjunction with the impurity and grain structure analysis.

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Figure 1