DC/pulse plating of copper for trench/via filling

A study was carried out to investigate the filling of line trenches and contact vias having an aspect ratio of 2:1 for sub-0.25 μm device manufacturing using both direct current (DC) and normal pulse plating of copper. Using a conventional CuSO4 electrolyte containing no additives, optimum DC plating conditions have been identified for the filling of the sub-0.25 μm line trenches and contact vias and plating was achieved successfully without any defects. However, complete filling only occurred after 70 s of plating under the optimum conditions. A two-step plating was carried out to fill the contact via and line trench at a faster rate (32 s), but trench filling was accompanied by voids and seam lines. Applying the normal pulse plating with an on-period of 6 ms or greater, complete line and via trench filling was obtained without any voids in 32 s of plating but a seam line was unavoidable in the line trenches. However, the seam line was eliminated successfully using a thermal reflow process at 400°C. The growth pattern of the pulse plated copper films deposited using 0.05 and 0.10 A/cm2 showed no significant difference. Small grained films are deposited uniformly across the line trenches and via holes when plated for 1 s. A slight build-up of the film thickness was observed with increasing plating time to 2 s, but without significant increase in the grain size. Grain growth was actually observed after plating for 5 s involving the coalescence of small grains. Finally, a further build-up in thickness and fill up of the trenches occurred after 10 s of deposition.