AlGe wafer bonding in ultra-high vacuum environment

Abstract

Despite it being known that native oxides impair diffusion processes during eutectic wafer bonding, the possibility of oxide removal combined with avoidance of oxide regrowth on the surfaces is not usually employed in AlGe eutectic wafer bonding. In the present work, the focus is on the impact of different process parameters like temperature, concentration, and oxide regrowth on the AlGe bonding layer, while processing inside ultra-high vacuum. Based on the results, it could be shown that the in situ AlGe eutectic bonding process enables acceptable bonding results already below the eutectic temperature Te = 423 °C, which would be required to liquefy the AlGe bonding layer at the eutectic concentration ce = 28.4 at. % Ge. Even if the AlGe system was not entering the liquid state, solid-state diffusion is sufficient to achieve a high bonding quality. The applied bonding force corresponds to a pressure of ∼0.16 MPa, which is much lower compared to the typical (ex situ) AlGe bonding process. In addition, it could be clarified that extreme variations of concentrations below and above ce were not as critical as expected. A good phase mixture as well as a low number of voids has been observed even if the resulting concentration did not fit ce. The main feature of the bonding system, which is oxide removal while suppressing the oxide re-growth, enhanced the quality of bonding result compared to the regular ex situ eutectic bonding approach.