The high-temperature requirement of Al-Ge eutectic bonding stands as a major obstacle in the wider acceptance for the hermetic sealing application among the micro-electro mechanical systems (MEMS) packaging industry, in particular for temperature sensitive devices. It has been demonstrated that the reduction in bonding temperature is feasible without compromising the hermeticity. The change in the mode of bonding from eutectic to solid-state diffusion did not have a dramatic impact on the bonding quality. However, this resulted in substantial increase in the bonding time. Shear strength too deteriorated as a result of the decrease in the thickness of the reaction interface. However, the shear strength still complied with the Military Standards (MIL). It has been confirmed that the hermetic seal could still be achieved without any solidification occurring at the interface. This is feasible since the inter-diffusion co-efficients of Al in (Ge) phase and Ge in (Al) phase are closer and are comparable to diffusion between solid-solution phases of identical metals like Au-Au, Cu-Cu, Si-Si bonding, which are generally used for such hermetic sealing application. Appropriate stacking mechanism for Al/Ge diffusion bonding is identified to overcome the limitations with respect to surface topography.
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