Effects of line width and liner material on the resistivity and grain growth of copper interconnects

The impact of Ru liner deposition methods on the resultant quality of EP Cu films is reported. Ruthenium liners from 5-17 nm in thickness were deposited by ALD, CVD and PVD; the ALD method resulted in Cu films with resistivities closest to the contribution from surface scattering alone. The best ALD Ru value shows a total resistivity which is only about 0.2 muO·cm more than the contribution from surface scattering, indicating large grain size. Next the influence of liner metal and thickness on grain growth in PVD Cu seed layers and EP Cu films on PVD Cu seed layers is reported. Copper seed resistivities were only 23% to 37% greater on 2 nm ALD Ru liners compared to 20 run PVD Ta liners even though the Ta was an order of magnitude thicker. One micron thick EP Cu on these more resistive seeds undergoes faster room temperature grain growth after deposition, but final resistivity of the EP Cu is not affected by liner metal. The surface scattering parameter for Ru was investigated. The results suggest a value of p = 0, consistent with two other reports, but the data are not conclusive. The effect of plating overburden on the grain size contribution to resistivity in Cu interconnects is reported. Overburden of 200 nm resulted in a 15% higher average initial resistivity compared to 800 nm overburden, while 300 nm overburden resulted in only 7% higher average initial resistivity compared to 800 nm. From a reliability viewpoint, small line width combined with high current density weakens the resistance to electromigration, and alloying of copper interconnects may be required. Therefore Cu alloy test structures and Cu on Cu alloy seed test structures were investigated. Three alloys, Cu-3% Zr, Cu-3% Ti, and Cu-3% In, were tested and compared in plating experiments. The Cu alloy structures showed significantly higher post-anneal resistivity than pure Cu, with the lowest value achieved being 5.3 muO·cm for 50 nm thick Cu-3% In alloy. Pure Cu on 10 nm Cu alloy seed, however, contributes to overall electrical resistivity increase of less than 0.5-1.0 muO·cm at 50 nm film thickness.