Simulation-based analysis of the Heat-Affected Zone during target preparation by pulsed- laser ablation through stacked silicon dies in 3D integrated System-in-Packages

The trends of 3D integration and System-in-Package (SiP) require the adaptation of target preparation methods for failure analysis of these complex integrated devices. Recent improvements in laser-based target preparation make laser cross-sections through several stacked silicon dies possible with remarkably small visible Heat-Affected Zones (HAZs). The distinct removal of Molding Compound (MC), silicon dies, and metal interconnects with a single laser source was demonstrated. The HAZ during laser preparation is extremely important. It must be determined whether the apparent passivation cracks are artifacts caused by the thermal load during laser preparation, or real defects in the device. In this paper, single laser impacts were used to investigate the HAZ. This approach is based on the penetration depth of the thermal wave for a specific pulse duration of the Diode-Pumped Solid-State (DPSS) laser system. Thermal simulations confirmed that the molten silicon cooled down to few degrees before the next laser pulse was deployed. The total energy that is coupled into the material during laser ablation is a very important parameter, but it is difficult to predict theoretically. The reason is that laser ablation combines various absorption mechanisms, electron excitation, rapid heating, ionization, sputtering, melting, vaporisation, and ejection. In order to estimate the amount of absorbed energy, the heating rate of the SiP was measured during laser ablation. Inverse analysis by simulation of the heating rate led to the percentage of energy coupled into the material during laser ablation. Once the amount of energy coupled into the device was estimated, the thermal simulation of a single laser-pulse impact provided the spatial and transient temperature profiles as well as the maximum temperatures within a certain distance from the irradiated area. These simulation results demonstrated clearly that defects beyond a certain distance of the laser cut edge were not caused by laser preparation. In conclusion, it is possible to predict the width of the HAZ by using a novel simulation method of laser ablation of packaging and chip materials. For that purpose, some crucial parameters such as the absorbed energy were determined.