共晶接合為微機電封裝的一種選擇，且密封性為一重要考量。利用高分子材料進行接合雖然有製程溫度低的優點，然而高分子材料本身結構存在許多排列鬆散的空間，許多的孔洞存在其中，外界中的微粒與水氣仍會進入系統內部中空腔室，這將會干擾這些脆弱的結構，導致系統於長期使用中的正常運作可靠度大幅降低，甚至損壞結構。由於金屬原子本身堆疊的緻密性以及受力所展現的延展性，且共晶點溫度小於純金屬的熔點，將能降低能源成本，避免在高溫製程中對元件造成損壞，因此在本實驗中，我們考慮利用鋁鍺共晶薄膜進行微機電封裝的接合動作，期望達到密封性封裝的效果。文獻中可得知共晶薄膜接合可有效地降低接合溫度，且經過拉伸測試與漏氣測試後知悉共晶薄膜接合有不錯的表現。
我們使用濺鍍的方式在矽基材上分別鍍上一層鋁與一層鍺，厚度各為100 nm，考慮氧化的問題，先鍍上鋁接著在不破真空的狀態下鍍上鍺，接著以200℃、400℃的溫度進行退火以觀察晶相的變化與擴散行為。計算可得200℃下交互擴散係數10^-22 m2/s，十天可擴散的長度約9nm，仍舊維持薄膜雙層結構；400℃下交互擴散係數為10^-20 m2/s，十天可擴散的長度約93nm，薄膜雙層結構消失且表面有許多的突起物，為釋放應力之結果。在這裡，鋁為主要擴散物種，由於濃度梯度的關係由下層擴散至試片上層，鍺相互聚集將中間的鋁擠出平面，形成雙層結構之突起物，試片表面由於相分離的關係分為富鍺區與富鋁區。
最後利用共晶組成的鋁鍺薄膜進行接合，鋁鍺共晶溫度為424℃，以高於共晶溫度100℃的溫度進行退火五小時，接合結果簡單地以紅墨水測試，接合區域沒有紅墨水滲入，初步證實鋁鍺共晶薄膜在密封性包裝上的可行性。

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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