Time- and Space-Resolved X-ray Diffraction for Stress Analysis in Semiconductor Devices

Since the conception of microelectronics, miniaturization has been a constant driver of innovation. The trend towards thinner wafers, finer interconnects, and higher switching frequencies exposes the devices to harsher mechanical conditions and pushes material requirements. This thesis investigates the role of residual stress in two industrially relevant cases: (i) the impact of residual stress on the strength of laser-diced Si dies, (ii) the thermomechanical fatigue of thick Cu interconnects found in high-power electronics. The results highlight the importance of using advanced characterization techniques to characterize stress at nanoscale length and millisecond timescales, which are typically inaccessible by conventional methods.
In the first case, cross-sectional X-ray nanodiffraction was employed to characterize stress in sub-micron thick recast layers of laser-diced Si dies. Those layers, formed by redeposition of the ablated material, consist of defect-rich polycrystalline Si. For the first time, the tensile residual stress in the recast layer of die sidewalls was quantified to exceed 290 MPa. The backside metallization was found to influence the microstructure, as metallic precipitates led to a significant refinement of the Si grain size. The intricate microstructure effectively tripled backside bending strength, demonstrating the pivotal role of grain boundary chemistry and morphology.
In the second case, high-frequency X-ray diffraction at a 20 kHz rate was used to characterize thermal stress in Cu metallizations during simulated overload pulses. Rapid heating from 25 to 525◦C at up to 106 K/s resulted in compressive stresses as high as −276 MPa. In situ dark-field X-ray microscopy studied early-stage thermomechanical fatigue due to multiple overload pulses. The formation of low-angle grain boundaries led to hierarchical subdivision of grains, while 2nd-order strain fields at high-angle grain boundaries increased and extended into the grain interior.
This thesis elucidates the intricate coupling between stress, microstructure, and mechanical failure in high-power electronics. By employing cutting-edge techniques with high spatial and temporal resolution to application-relevant problems, novel insights into the origin of stress and device failure have been gained. These insights will help design more robust devices for the next generation of high-power electronics.