Passivated Interconnect Lines Research Articles

Reduction of annealed-induced wafer defects in dual-damascene copper interconnects

Interconnects in very large scale integration (VLSI) chips are susceptible to failure due to mechanical stress in passivated interconnect lines. These mechanisms play a collective role for intensive research of thermal stability for Cu interconnects reliability in CMOS technologies. This paper presents capabilities and performance on samples annealed using furnace vs He in situ anneal and in-line technique developed to reduce total defect count.

Interfacial thermal stress analysis of an elliptic inclusion with a compliant interphase layer in plane elasticity

Stresses induced by thermal mismatch are known to be a major cause of failure in a wide variety of composite materials and devices ranging from metal–ceramic composites to passivated interconnect lines in integrated circuits. One of the most effective procedures used to reduce these thermal stresses is the addition of a compliant intermediate or interphase layer between the different material components. This paper is concerned with the interfacial thermal stress analysis of an elliptic inclusion embedded within an infinite matrix with uniform change in temperature. A compliant interphase layer is assumed to occupy the region between the inclusion and the matrix. This interphase layer is modeled as a spring layer with vanishing thickness (henceforth referred to as the interface between the inclusion and the matrix). Its behavior is based on the assumption that tractions are continuous but displacements are discontinuous across the interface. Complex variable techniques are used to obtain infinite series representations of the thermal stresses which, when evaluated numerically, demonstrate how the peak interfacial thermal stresses vary with the aspect ratio of the inclusion and the parameter h describing the interface. In addition, and perhaps most significantly, for different aspect ratios of the elliptic inclusion, we identify a specific value (h∗) of the interface parameter h which corresponds to the maximum peak thermal stress along the inclusion–matrix interface. Similarly, for different aspect ratios, we identify a specific value of h (also referred to as h∗ in the paper) which corresponds to the peak maximum thermal strain energy density along the interface (J. Appl. Mech. 57 (1990) 956–963). In each case, we plot the relationship between the new parameter h∗ and the aspect ratio of the ellipse. This gives significant and valuable information regarding the failure of the interface using two established failure criteria.

Stress Analysis of Thermal Inclusions With Interior Voids and Cracks

Thermal mismatch induced residual stresses are identified as one of the major causes of voiding and failure of some critical components in electronic packaging, such as passivated interconnect lines and isolation trenches. In this paper, a general method is presented for thermal stress analysis of an embedded structural element in the presence of internal or nearby voids and cracks. Here, the elastic mismatch between dissimilar materials is ignored. Hence, the embedded structural element is modeled as a thermal inclusion of arbitrary shape surrounded by an infinite elastic medium of the same elastic constants. Thermal stresses are caused by thermal mismatch between the inclusion and the surrounding material due to a uniform change in temperature. With the present method, the problem is reduced to one of an infinite homogeneous medium containing the same voids and cracks, subjected to a set of remote stresses determined by the geometrical shape of the thermal inclusion. In particular, the remote stresses are uniform when the thermal inclusion is an ellipse. The method gives an elementary expression for the internal stress field of a thermal inclusion with a single interior void or crack. Several examples of practical interest are used to illustrate the method. The results show that an internal void or crack can significantly change stress distribution within the inclusion and gives rise to stress concentration around the void or crack. [S1043-7398(00)00303-0]

Passivated Interconnect Lines: Thermomechanical Analysis and Curvature Measurements

AbstractIt is well known that curvature measurements may be used to obtain volume averaged stresses in thin continuous films and unpassivated lines without knowledge of the material properties of the film or lines. However, recently a method was presented which makes it possible to use curvature measurements also for the determination of volume averaged stresses in passivated lines. Since the problem is statically indeterminate the method requires knowledge of the material properties of the lines and passivation. The sensitivity of the method to uncertainties in material properties and curvature data is here investigated by utilizing the finite element method for anisotropic Cu or Al lines embedded in SiO2 passivation. Furthermore, the method is extended to cover the case of different stress-free temperatures for the lines and passivation respectively.

Effect of interphase layers on thermal stresses within an elliptical inclusion

Thermal mismatch induced stresses are identified as the major cause of failure in a wide variety of materials and devices, ranging from metal–ceramic composites to passivated interconnect lines in integrated circuits. To reduce thermal stresses, an effective method is to add an intermediate layer, with appropriate thermal expansion coefficient, between the components of dissimilar materials. This paper gives a general analysis of the effects of the intermediate layers on thermal stresses within an elliptical inclusion, with particular emphasis on the role of thermal mismatch. The exact closed-form solution is obtained for stress field. One of the unique features of the present model is that the thermal stresses within the elliptical inclusion are uniform, making simple formulas available for quantitative analysis of the effects of the interphase on thermal stresses within the inclusion. It is found that the interphase layers have a strong effect on the deviatoric stress and a moderate effect on the mean stress within the inclusion. In particular, the effect of the interphase on the mean stress is sensitive to both the aspect ratio of the elliptical inclusion and the elastic mismatch between the inclusion and the surrounding materials, but not for the deviatoric stress within the inclusion. To reduce the thermal stresses within the inclusion, the optimum thermal expansion coefficient of the interphase is not necessarily between those of the inclusion and the matrix. However, if the design goal is to reduce the thermal stresses within both the inclusion and the interphase layer, the optimum interphase should have an intermediate thermal expansion coefficient between those of the matrix and the inclusion.

The Stress Change in Passivated Al Lines Due to the Reaction Between Ti and Al

AbstractThe thin film reaction between Ti and Al-0.5%Cu to form TiAl3 is common in the microelectronics industry. In this paper the stress changes in Al-0.5%Cu films at elevated temperatures during the reaction are measured. The changes are measured in blanket films as well as in passivated interconnect lines. Results show that in blanket films the Al-0.5%Cu does not experience any stress change due to the reaction. However in passivated lines, where the layers are not allowed to relax in the normal direction, tensile stresses build up in the Al-0.5%Cu due to the volume shrinkage that happens when these films react.

Void Nucleation in Passivated Interconnect Lines: Effects of Site Geometries, Interfaces, and Interface Flaws

Stress driven nucleation of voids in passivated aluminum interconnect lines is analyzed within the context of classical nucleation theory. A discussion of sources of tensile stress in such lines leads to an upper limit of 2 GPa. Calculations suggest that even at this high stress, nucleation rates are far too low to account for observed rates of voiding. Void formation at a circular defect at the line/passivation interface is then considered. In this case, a flaw on the order of nanometers in size may develop into a void under the imposed stress. These results strongly suggest that void nucleation in aluminum interconnect lines can be controlled by eliminating defects in the line/passivation interface.

An Analysis of Void Nucleation In Passivated Interconnect Lines Due to Vacancy Condensation and Interface Contamination

AbstractNucleation of voids due to vacancy condensation in passivated aluminum lines is analyzed within the context of classical nucleation theory. A discussion of sources of hydrostatic tensile stress in such lines provides a reasonable upper limit of 2 GPa. The void nucleation rate is then calculated at various sites within the line. Results suggest that nucleation rates are far too low to account for observed rates of voiding. Void nucleation at a flaw at the line/passivation interface is then considered as an alternative nucleation mechanism. Such flaws may be created by contaminants introduced during fabrication of the line. In this case, nucleation is feasible at greatly reduced stresses, well within the observed values. Furthermore, a simple model of void growth indicates that a fast atomic transport path, such as a grain boundary, must intersect the void for an appreciable growth rate. These results suggest that void nucleation in aluminum interconnect lines occurs at flaws at the sidewall of the line and that stress-induced and electromigration-induced voiding can be controlled by eliminating interfacial contamination.

Analysis of stress-induced void growth mechanisms in passivated interconnect lines

Simple theoretical models have been used to investigate four possible mechanisms of void growth in interconnect lines with a bamboo grain structure. The mechanisms considered are: creep deformation by dislocation motion; grain-boundary diffusion; diffusion along the interface between the line and the surrounding passivation; and void growth by lattice diffusion. The results show that all four mechanisms may cause the void to grow, but any one mechanism operating alone gives rise to slow or negligible rates of growth; however, plastic creep flow and grain-boundary diffusion may act in a cooperative manner, which causes rapid void growth. It is shown that the extent of coupling between diffusion and creep increases as the linewidth is reduced. Finally, the numerical calculations are used to estimate the time required for the line to fail, as a function of temperature and linewidth. The predictions are shown to be consistent with observations reported in the literature.