Reactive multilayer foils for MEMS wafer level packaging

We present a novel room temperature bonding technique using reactive multilayer Ni/Al foils as local heat sources to heat intermediate adhesion layers and thus bond silicon wafers to form wafer level package. Exothermic reactions in Ni/Al multilayer foils were investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). XRD measurements showed that the dominant product after the exothermic reaction was ordered B2 AlNi compound. The heat of reaction was calculated to be -57.9 kJ/mol. Two bonding approaches were realized. One used AuSn solder as an intermediate layer to bond two silicon wafers together; the other approach conducted bonding between Parylene-C and silicon wafers with a thin Parylene-C coating. Either silicon or Parylene-C was torn when the bond was forcefully broken, indicating a strong bond was achieved. Moreover, leakage test in isopropanol alcohol (IPA) showed that the joints possessed good hermeticity to liquid. Numerical simulation results demonstrated that both localized heating and rapid cooling occurred during the bonding process, which made reactive foil bonding an ideal method for MEMS wafer level packaging applications.