Failure analysis of fabrication process in hermetic wafer-level packaging for microbolometer focal plane arrays

Wafer-level hermetic packaging is attractive for microbolometer-based uncooled infrared cameras as it enables low manufacturing cost and high-volume production of the sensors. A large package is designed to accommodate highly sensitive microbolometers with a high pixel count. This paper focuses on the study of failure mechanisms related to fabricating such large hermetic packages at wafer-level. We bonded wafers containing-12 mm x 12 mm dies using Cu-Sn solid-liquid interdiffusion (SLID) bonding and subsequently analysed the possible causes for failures. A time-dependent cap deflection study was carried out on the success-fully bonded dies to evaluate the vacuum condition inside the package. The main causes for failures determined by this study include insufficient bonding pressure, non-uniform electroplating, voids in the sealing frame, and local cracks on the cap wafer with cavity. These factors contribute to a low dicing yield and loss of vacuum. A high dicing yield were obtained by Cu electroplating with 5 mA/cm2 current density, & GE;4 MPa bonding pressure, and & GE;200 mu m cap thickness. In the cap deflection study, 7 out of 19 dies in a wafer maintained high vacuum after 178 days as indicated by limited decline in the cap deflection (-1-6 %), which shows the potential of this wafer-level packaging for obtaining long-term reliability.