Electromigration-Induced Plastic Deformation in Passivated Metal Lines

We have used scanning white beam x-ray microdiffraction to study microstructural evolution during an in-situ electromigration experiment on a passivated Al(Cu) test line. The data show plastic deformation and grain rotations occurring under the influence of electromigration, seen as broadening, movement, and splitting of reflections diffracted from individual metal grains. We believe this deformation is due to localized shear stresses that arise due to the inhomogeneous transfer and deposition of metal along the line. Deviatoric stress measurements show changes in the components of stress within the line, including relaxation of stress when current is removed. Electromigration (EM) is a phenomenon that occurs when extremely high current densities (j -lo6 A/cm*) lead to mass transport of metal within integrated-circuit metallizations.i Failure of the interconnect can be caused by open circuit voiding or short circuit extrusions of the metal. The evolution of stress caused by EM in metallic interconnects is an important topic in microelectronics reliability.* Large stresses can develop in the line because of the transport of metal in a confined space. A great deal of research has been conducted in an attempt to understand the role of stress and stress gradients during EM and several models have been proposed.3T4.5 Experimental verification of these models has proven difficult due to the challenge of measuring stress in passivated interconnect structures with the necessary spatial resolution. With recent advances in synchrotron and x-ray optics technology, x-ray microbeams have proven useful in the study of EM. X-rays are ideal for interconnect studies, as they can be focused on the order of the grain size and can penetrate any dielectric covering the line, unlike electron beams, which are only sensitive to the sample surface. Several groups have recently reported results using various x-ray microbeam techniques.6Y7Y8 In this letter, we report results using scanning white beam x-ray microdiffraction, which allows for mapping the complete orientation and deviatoric stress/strain tensor of micron-scale grains within a passivated interconnect line. Additionally, the constituent Laue reflections for a given gram can yield information about plastic deformation that may occur during EM. This experiment was conducted at Beamline 7.3.3 at the Advanced Light Source synchrotron in Berkeley, CA. A detailed description of the beamline is available in a recent article.’ A white x-ray beam (6-14 keV) is focused to a spot size of 0.8 x 0.8 pm