Cu-Sn Wafer Level Bonding for Vacuum Encapsulation of Microbolometer Focal Plane Arrays

Abstract

Wafer level vacuum encapsulation for wafers with integrated and released sensitive microelectromechanical structures has been achieved by making use of Cu-Sn solid liquid interdiffusion bonding. By using a Sn layer as oxidation barrier for the Cu underneath, the bonding surface does not require pre-cleaning or use of any flux agent prior to, or during bonding. With a tailored temperature and pressure bonding profile, the amount of Sn squeeze-out is reduced. Cross sections through the bonded frames were used to investigate the formation of Cu-Sn intermetallic compounds. The strength of the joints was measured using a shear test, which resulted in an average value of 35MPa. Further temperature cycling of bonded dies does not result in any reduction in bonding yield or shear strength. The concentric interference fringes revealed by IR pictures of the bonded wafer pairs demonstrate that the cap on each device is deflected towards the bottom wafer, confirming that the pressure in the bonded cavities is low.