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

Abstract

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 × 12 mm dies using CuSn solid-liquid interdiffusion (SLID) bonding and subsequently analysed the possible causes for failures. A time-dependent cap deflection study was carried out on the successfully 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, ≥4 MPa bonding pressure, and ≥200 μm 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.