| dc.description.abstract | Eutectic bonding is one of the methods among MEMS packaging, and hermetic sealing is an essential consideration. Although polymer material offers a low temperature progress, however, particles and moisture would penetrate through the loose molecular structure and get into the inner tiny chamber. It would destroy or disturb these fragile structures and therefore decrease the reliability for long period working. Due to the dense package of atoms and the ductile property, the alloy of Al-Ge may provide a new option for the application on MEMS hermetic sealing. Alloy of eutectic composition would melt at the temperature much lower than pure metal, it would effectively decrease the energy cost and avoid the mechanical damage to structure during the process. Therefore, the eutectic Al-Ge thin films could effectively lower the bonding temperature and show good performance in bonding quality according to the results of pull test and leak tests in previous studies.
In this experiment, the Al-Ge thin films were deposited on silicon wafer by sequential sputtering at room temperature. The thicknesses were 100nm for each layer. Ge layer was deposited after Al layer without venting. After deposition, the Al-Ge films were annealed at 200℃ and 400℃ in vacuum furnace to observe the diffusion behavior and the crystallization. We could get the diffusivity of interdiffusion from calculation, about 10-22 m2/s for 200℃ and 10-20 m2/s for 400℃. The films could maintain the bi-layer structure at 200℃ for the diffusion length is only 9.2nm after 10 days. However, the bi-layer structure disappeared after annealing at 400℃ for the diffusion length is about 93nm after 10 days. Also, there were a lot of extrusions on the surface due to relieving the inner stress. Here, Al is the dominant diffusion species and diffused to the surface due to the large concentration gradient. Ge atoms on the surface started to segregate to relieve the stress. Small Al-rich pellets surrounded by Ge-rich region were extruded out of the plane to further release the residual stress. The matrix area could be divided into Al-rich area and Ge-rich area due to phase separation phenomena.
At the final step, we use eutectic Al-Ge thin film to execute the bonding experiment at 525℃ for 5 hours, 100℃ higher than the eutectic temperature. The bonding result is examined using a easy test, the red ink test, to investigate the ability of defense against the red ink. The red ink didn’t permeate into the bonded area. And it could offer a rough proof that Al-Ge eutectic bondimg had achieved the goal of hermetic sealing.
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