Diffusional creep in damascene Cu lines

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Abstract
Damascene Cu lines are increasingly being used as electrical interconnects in microelectronic integrated circuits. During the manufacturing process, Cu interconnects are subjected to thermal cycling that leads to complex stress states, which affect their reliability. While a realistic assessment of the impact of thermal stresses on the reliability should take into account the inelastic behavior of the damascene Cu lines, unfortunately, little is known about their mechanical behavior. The goal of this work was to identify the physical mechanisms responsible for the inelastic behavior of damascene Cu lines, and to assess the effects of the varied dimensions and passivation layers. The curvature changes caused by thermal cycling of wafers with damascene Cu lines were measured in the temperature range from −80 to 450 °C. At low temperatures, the deformation in the lines is predominantly elastic, but becomes inelastic above 200 °C. Our experimental results indicate that diffusion-controlled creep is the dominant inelastic mechanism at high temperatures. A TaN capping layer was found to suppress inelastic deformation, presumably by preventing diffusion of Cu between the free surface and the grain boundaries. This effect suggests a simple method for comparison of the effectiveness of different capping layers (e.g., TaN, Ta, Si3N4) in inhibiting Cu interfacial diffusion.