BEoL Stack Failure Mode Evaluation Utilizing Micromechanical Testing and FEM Modelling

In this work, the results of a mechanical BEoL (back end of line) stack failure mode evaluation study of a high-end microchip are presented. Deploying micromechanical testing approaches enables the determination of different failure modes and, in some cases and with the right methods, the identification of the location of damage initiation as well as an estimation of damage propagation. However, it is not possible to determine the mechanical stress conditions inside the BEoL stack which cause these damages experimentally. To obtain this information, it is required to develop and deploy an FEM (Finite Element Method) simulation model. In this work, the experimental results of the application of two micromechanical loading methods to induce three different failure modes into a BEoL stack are presented. These results have partially been introduced in the previous studies [1], [2], and [3] but are put into perspective and aligned with FEM simulation results to obtain a more holistic understanding of the occurring damage modes and the related mechanical conditions. The FEM simulations have been utilized to quantify the critical values and location of mechanical stresses inside of the BEoL stack which result in the mechanical failure modes. A Cu-pillar shear approach has been deployed as well as a soldering and tensile loading approach. These methods have been introduced in a detailed manner in [4]. Sub-critical modifications of these Cu-pillar loading experiments as described in [2] and [4] in which AE (acoustic emission) was utilized as a damage indicator, have been deployed as well to determine the damage initiation location and progression through the BEoL stack. Based on this combined method consisting of the experimental approaches and the related FEM simulations, mechanical failure models of the investigated BEoL stack could be derived. These include the location of damage initiation and the related mechanical stresses as well as an estimated damage progression. A specific design optimization is suggested based on the results presented in this study and evaluated deploying an additional FEM simulation with the mechanical loading conditions of the tensile failure mode.