Advanced interconnect technologies such as Through Silicon Vias (TSV) have become an integral part of 3-D integration. International Technology Roadmap for Semiconductors (ITRS) has identified a need for an in-line metrology for characterizing voids in TSV structures. In this paper, we describe a laser-based acoustic technique in which a short laser pulse generates broadband acoustic waves that propagate in the TSV structure. An optical interferometer detects the surface displacement caused by the acoustic waves reflecting within the structure as well as other acoustic waves traveling near the surface that has information about the structure dimensions and irregularities. Sensitivity of the technique to detect various types of voids has been confirmed by performing cross-section microscopy. Measurements typically take few seconds per site and can be easily adopted for in-line process monitoring. The technique has also demonstrated capability for measuring copper pillar stacks, characterizing bonding voids and delamination. INTRODUCTION Microelectronics industry trends have continued to move rapidly towards 3-D integration of semiconductor devices and the driving force remains the need for smaller, faster devices with optimized performance, enhanced data transfer speeds, minimum transmission loss, while maintaining reliability and meeting cost targets. TSV technology has entered the mainstream for 3D ICs as they support heterogeneous integration of logic and memory devices resulting in significant improvements in performance. Depending on the application, a " Via-first " or " Via-last " approach may be used in manufacturing. Nevertheless, the process is complex and filling the high aspect ratio vias with copper is one of the most challenging and expensive steps of the fabrication process. Achieving void-free copper fill is critical to avoiding reliability problems and improving yield. Void formation may occur at the bottom of the vias, along the seamline and at the top of the vias and are dependent on the plating chemistry and approach used [1].