Abstract
The drive to reduce the interconnect pitch and increase the number of connections for packaging in mobile devices has led to the development of copper pillar bumps. The key drivers for the adoption of copper pillars are improved performance, reduced form factor, and lower cost. In this article, we present a laser-based acoustic technique for the characterization of multilayer pillars. This noncontact technique has a high sensitivity for materials characterization with micron-scale spatial resolution. Absorption of laser light causes excitation of elastic waves that propagate through the pillar and are reflected by the pillar walls, exciting vibrational modes in the structure. We have demonstrated that our approach is sensitive to the thicknesses of individual layers in bilayer and trilayer copper pillar stacks. Focused ion beam scanning electron microscopy (FIB-SEM) has been used to optimize the model and to validate the accuracy of the technique.
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