Evaluation of Low-k Integration Integrity Using Shear Testing on Sub-30 Micron Micro-Cu Pillars

Mechanical integrity of low-k dielectric films remains a quality and reliability challenge for devices using advanced silicon nodes. In wafer fabs, while great efforts are made in controlling and monitoring individual processing steps, the overall mechanical quality of a particular device is not often effectively monitored. Defects such as interfacial delamination may only manifest themselves during system assembly processes or in-field operation, bringing significant disruption and impact onto production and product quality. As silicon sizes and package sizes continue to grow, chip-packaging interaction becomes more significant, and the risk of low-k related failures increases as a result. Particularly for 3D and 2.5D devices, the complexity of chip stacking makes it important to have a quantitative assessment of dielectric quality for both yield and ongoing reliability management purposes. The adoption of micro-Cu pillars on 2.5D and 3D devices provides an opportunity for direct measurement of integration quality. If shear testing can be performed on individual micro-Cu pillars, responses from such testing can be analyzed and quantified as a direct measurement of integration quality. Furthermore, such testing can be performed on a specific device of interest and on specific locations on a die, which makes it possible to use this technique as a product quality control method. In this paper we will report results from shear testing on sub-30 micron micro-Cu pillars. Data from multiple wafers, dies, and bump locations will be reported. Responses such as load-distance curves and maximum fracture load are analyzed. In addition, multi-level finite element models are developed to simulate the shear test. Locations of stress concentration will be identified and compared with fracture interfaces from the shear test. Responses to changes in properties of the dielectric layers will also be investigated, which provides insight into the variations in shear strength observed in real-life shar testing.