Robustness of Large-Size Vacuum Sealed Packages for Microbolometer Array

Microbolometers are the core detectors of uncooled thermal sensors. These detectors require a high vacuum environment (<1 Pa) to function, necessitating hermetic packaging where the robustness of the packages is crucial to maintain such a high vacuum over time. This study investigates the sealing frame integrity of large-size ( $\sim 12\times 12$ mm) wafer-level Cu-Sn SLID packages using a time-dependent cap deflection study and exposing them to humidity and thermal shock test environments. Cap deflection measurements at the wafer level indicate that the dies maintained a stable vacuum for more than 13 months after bonding before they were exposed to harsh environments. The Cu-Sn SLID packages display resilience to corrosion effects, with 93% of the dies passing the humidity test. In contrast, all dies failed the thermal shock test due to vertical cracks in the Cu/Cu3Sn/Cu bondline. These vertical cracks are primarily found to propagate through voids in the Cu3Sn, where the stresses are assumed to be the largest. To mitigate vertical crack formation and enhance long-term sealing frame integrity, void formation must be minimized. Strict requirements are thus put on the electroplating process to avoid contamination, demanding careful monitoring of the bath conditions and optimization of the electroplating parameters.