Electroplated copper exhibits some surprising changes at room temperature in sheet resistance, stress, and microstructure. This behavior, now known as self-annealing, is shown here to be intimately linked to the composition of the plating bath and the resulting incorporation of organic additives in the Cu layer. Their addition is a necessary condition for self-annealing to occur, but slows down the process for higher concentrations. The phenomenon also depends critically on film thickness, showing an accelerated transformation when film thickness increases. This dependence is explained in terms of a very rapid primary crystallization from the top surface down just after deposition, followed by a slower lateral recrystallization producing large secondary grains. The stress and sheet resistance during recrystallization are identified as two noncorrelated variables.

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When a Cu surface is exposed to a clean room ambient, a surface layer containing Cu 2 O, CuO, Cu͑OH͒ 2 , and CuCO 3 is formed. Thermal treatment in a vacuum combined with hydrogen plasma can remove this layer. Water and carbon dioxide desorb during the thermal treatment and the hydrogen plasma reduces the remaining Cu oxide. Ellipsometric, x-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectroscopy analyses indicate that the mechanism of interaction of the H 2 plasma with this layer depends on temperature. When the temperature is below 150°C, H 2 plasma cannot completely reduce Cu oxide. Hydrogen diffuses through the oxide and hydrogenation of the Cu layer is observed. The hydrogenated Cu surface has a higher resistance than a nontreated Cu layer. The hydrogen plasma efficiently cleans the Cu surface when the substrate temperature is higher than 150°C. In this case, hydrogen atoms have enough activation energy to reduce Cu oxide and adsorbed water forms as a byproduct of Cu oxide reduction. When the wafer temperature is higher than 350°C, the interaction of the Cu film with hydrogen and residual oxygen is observed.

Knowledge of the geometry effect on impurity incorporation and grain growth in narrow lines is important for reducing copper line resistivity. In this paper, we investigate impurity incorporation in narrow lines with time-of-flight secondary ion mass spectroscopy. We also study the influence of linewidth, trench depth, pattern density, and overburden on copper grain growth in reduced dimensions. The concentration of chlorine and carbon is found to increase with decreasing linewidth, while the concentration of sulfur is close to the detection limit. This effect contributes to copper superfilling and is consistent with the curvatureenhanced accelerator coverage model. Copper self-anneal slows down as linewidth decreases, although an opposite trend is observed for lines below a width of about 300 nm. Impurity incorporation and geometric constraint retard copper grain growth in narrow lines, resulting in an inverse relationship between copper line resistivity and geometry. However, copper overburden, which has a much larger grain size, can enhance grain growth in narrow lines depending on the line geometry. Reducing impurities and balancing trench depth and copper overburden can be used to reduce the resistivity of narrow copper lines.

As an interconnect material, copper has the disadvantage of not forming self-limiting oxides, which can negatively affect device performance and reliability. Undesired oxide layers need to be removed by in situ cleaning, before the copper is subjected to subsequent depositions. We have used ethyl alcohol (C 2 H 5 OH) as a vapor phase reducing agent to remove copper oxides formed on electroplated copper films upon exposure to the ambient. Spectroscopic ellipsometry has been used to monitor the reduction process in situ. Ex situ characterization using X-ray photoelectron spectroscopy and atomic force microscopy support in situ measurements. While oxide removal can be achieved at temperatures as low as 130°C, independent of oxide layer thickness and composition, it occurred more efficiently at 200°C, showing compatibility with the low temperature (Ͻ400°C) processing requirements of low dielectric constant materials. The initial reaction involves the reduction of Cu 2ϩ to Cu ϩ species followed by a second phase consisting of Cu ϩ conversion to elemental copper, producing a clean metal surface. Reduction of Cu 2ϩ to Cu ϩ species is the rate-limiting step as evidenced by enhanced sensitivity to the reaction temperature.