GOALI: Measurement & Modeling of Heterogeneous Nanoscale Deformation of Copper-Polyimide High Density Interconnect Structures Subjected to Thermal Cycling

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9631319 Gross High density interconnect (HDI) structures based on the Cu-polyimide system have exhibited improvements in signal propagation rate relative to the Al-SiO2 system. However, the large mismatch in out-of-plane thermal expansion coefficients generates high interfacial stresses during processing that may adversely affect the reliability of the structure. This problem will become more severe as the number of wiring levels increases. Recent experimental evidence has shown that the elastic properties and thermal expansion of the polyimides are a function of structure size for 1 (m widths and below. Additional experimental evidence suggests that the inelastic deformation of the Cu in these structures is localized on grain boundaries and at the interface between the Cu and the polyimide. Furthermore, plastic deformation by dislocation glide has been shown to be negligible for the relevant temperature range. Since the feature size is on the order of the Cu grain size, finite element models based on continuum models of inelastic deformation will not accurately describe the stresses in these HDI structures. The goal of the proposed analytical and experimental program is to develop and evaluate strategies to model heterogeneous, inelastic deformation based on diffusive transport in these structures. Specifically, inelastic deformation will be confined to the grain boundaries and bimaterial interfaces. The interface and grain boundary constitutive models will be based on the mechanisms of Coble creep and grain boundary sliding. Deformation in the grain interiors and in the polyimide will be modeled as linearly elastic (with temperature dependent elastic constants and coefficients of thermal expansion). A hot stage scanning probe microscope will be used to measure the amount and rate of interface and grain boundary sliding as a function of temperature. Measurement of the buildup of material at grain boundaries and interfaces will be used to quantify rates of material transport for the Coble creep mechanism. A second, related goal is to develop strategies to measure the extent and change of properties of the polyimides caused by damage from the etch used to define the pattern for the conductor lines. The trend toward reducing the size of features in HDI structures requires the development of strategies to model thermomechanical deformation that take into account the heterogeneous nature of deformation and recognize that deformation by diffusive transport will be of first order significance for nanostructures. Understanding these effects and documenting their magnitude will have broad significance to the entire microelectronics industry. ***