Mechanical resistance of patterned BCB bonded joints for MEMS packaging

Adhesive wafer bonding is an interesting key technology for heterogeneous integration of films or devices coming from different technologies in the view of 3D heterogeneous integration of NEMS/MEMS. An original wafer level MEMS packaging process consists in the transfer of nickel caps from a temporary wafer to the target wafer carrying the devices to be packaged. In this process the polymer used as sealing ring is submitted to a first mechanical loading as the film cap is removed from the donor wafer. It is therefore important to have quantitative data about the mechanical resistance of the bond in order to ensure its integrity after the fabrication process. In this study, wedge insertion method and four-point bending test are used to characterize bonds of silicon wafers without and with a nickel interlayer. The adhesive polymer is a photosensitive benzocyclobutene (BCB) resin patterned into lines either parallel or perpendicular to the crack growth direction. The measured fracture energies completed by observations of the fracture surfaces allows the identification of various parameters influencing the resistance of the stack. A first parameter is the adhesive lines orientation: when BCB lines are perpendicular to the crack growth direction higher adhesion energies are measured than when they are parallel to this direction. A second parameter is the BCB curing atmosphere: adhesive bonds that experienced a curing treatment in N"2 gas have higher fracture resistance than those cured in vacuum. Finally, the addition of a nickel film between the silicon wafer and the BCB adhesive embrittles the mechanical resistance of the bond because Ni has a poor adhesion both on silicon and on BCB.